SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

Form 10-K/A

(Amendment No. 1)

(Mark One)

For the fiscal year ended December 31, 2010

OR

For the transition period from                     to                    

Commission File No. 001-32331

ALPHA NATURAL RESOURCES, INC.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code:

(276) 619-4410

Securities registered pursuant to Section 12(b) of the Act:

Securities registered pursuant to Section 12(g) of the Act:

None

Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act.    Yes  x    No  ¨

Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Act.    Yes  ¨    No  x

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days.    Yes  x    No  ¨

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§ 232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files).    Yes  x    No  ¨

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained herein, and will not be contained, to the best of registrant’s knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K.    x

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act.

Indicate by check mark whether the registrant is a shell company (as defined in Exchange Act Rule 12b-2).    Yes  ¨    No  x

The aggregate market value of the Common Stock held by non-affiliates of the registrant on June 30, 2010, was approximately $2.7 billion based on the closing price of the Company’s common stock as reported that date on the New York Stock Exchange of $33.87 per share. In determining this figure, the registrant has assumed that all of its directors and executive officers are affiliates. Such assumptions should not be deemed to be conclusive for any other purpose.

Common Stock, $0.01 par value, outstanding as of April 20, 2011 – 120,867,566 shares.

DOCUMENTS INCORPORATED BY REFERENCE

Part III incorporates certain information by reference from the registrant’s definitive proxy statement for the 2011 annual meeting of stockholders (the “Proxy Statement”), which was filed within 120 days after the close of the registrant’s fiscal year ended December 31, 2010.

Explanatory Note

In response to comments raised by the Staff of the Securities and Exchange Commission, Alpha Natural Resources, Inc. (the “Company”) is filing this Amendment No. 1 to the Company’s Annual Report on Form 10-K for the year ended December 31, 2010 to revise Item 2 to include separate quantification of proven reserves and probable reserves. The complete text of Item 2 is set forth herein, including those portions of the text that have not been amended from the original Form 10-K, and replaces Item 2 in the original Form 10-K in its entirety. In addition, due to an inadvertent omission, the Company has revised Item 9B to add a sentence referring to and incorporating by reference Exhibit 99.1. All other Items of the original Form 10-K are unaffected by this Amendment, and those Items have not been included in this Form 10-K/A.

The information contained in this Form 10-K/A does not reflect events that have occurred on or after February 25, 2011, the filing date of the original Form 10-K, or otherwise modify or update the disclosure presented in the original Form 10-K, except as described above.

Accordingly, this Form 10-K/A should be read in conjunction with the original Form 10-K, as well as any filings the Company has made with the Securities and Exchange Commission since the filing of the original Form 10-K.

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Coal Reserves

We periodically retain outside experts to independently verify our estimates of our coal reserves. “Reserves” are defined by the Securities and Exchange Commission (“SEC”) Industry Guide 7 as that part of a mineral deposit which could be economically and legally extracted or produced at the time of the reserve determination. “Proven (Measured) Reserves” are defined by SEC Industry Guide 7 as reserves for which (1) quantity is computed from dimensions revealed in outcrops, trenches, workings or drill holes; grade and/or quality are computed from the results of detailed sampling and (2) the sites for inspection, sampling and measurement are spaced so closely and the geologic character is so well defined that size, shape, depth and mineral content of reserves are well-established. “Probable reserves” are defined by SEC Industry Guide 7 as reserves for which quantity and grade and/or quality are computed from information similar to that used for proven (measured) reserves, but the sites for inspection, sampling and measurement are farther apart or are otherwise less adequately spaced. The degree of assurance, although lower than that for proven (measured) reserves, is high enough to assume continuity between points of observation.

Information about our reserves consists of estimates based on engineering, economic and geological data assembled and analyzed by our internal engineers, geologists and finance associates, as well as third party consultants retained by us. We periodically update our reserve estimates to reflect past coal production, new drilling information and other geological or mining data, and acquisitions or sales of coal properties. Coal tonnages are categorized according to coal quality, mining method, permit status, mineability and location relative to existing mines and infrastructure. Further scrutiny is applied using geological criteria and other factors related to profitable extraction of the coal. These criteria include seam height, roof and floor conditions, yield and marketability.

Prior to Old Alpha’s initial public offering in 2005, a third party consultant was retained to perform reserve estimates in November 2004. Since November 2004, a third party consultant has been retained to verify reserves for our major acquisitions, which include the Callaway, Progress Fuels, Mingo Logan Ben’s Creek Complex, and Foundation acquisitions, as well as to conduct ongoing reserve updates, on an annual basis, for specific properties that have undergone substantial modification to the reserve base. Properties that have undergone insignificant or no changes since the original assessment in November 2004 have been carried forward without re-evaluation.

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These reviews include the preparation of reserve maps and the development of estimates by certified professional geologists based on data supplied by us and using standards accepted by government and industry, including the methodology outlined in U.S. Circular 891. Reserve estimates were developed using criteria to assure that the basic geologic characteristics of the reserve (such as minimum coal thickness and wash recovery, interval between deep mineable seams and mineable area tonnage for economic extraction) were in reasonable conformity with existing and recently completed operation capabilities on our properties.

We estimate that, as of December 31, 2010, we owned or leased total proven and probable coal reserves of approximately 2,253.5 million tons. We believe that we have sufficient reserves to replace capacity from depleting mines for the foreseeable future and that our current reserves are one of our strengths. We believe that the current level of production at our major mines is sustainable for the foreseeable future.

Of the 2,253.5 million tons, approximately 1,155.0 million tons were assigned reserves that we expect to be mined in future operations. Approximately 1,098.5 million tons were unassigned reserves that we are holding for future development. All of our reserves in Wyoming are assigned. As of December 31, 2010, we had unassigned reserves in Pennsylvania, West Virginia, Virginia/Kentucky and Illinois of 649.7 million tons, 242.1 million tons, 178.1 million tons and 28.6 million tons, respectively.

Approximately 64% of our reserves are classified as high Btu coal (coal delivered with an average heat value of 12,500 Btu per pound or greater) and are located in Pennsylvania, West Virginia, and Virginia/Kentucky. Approximately 64% of our reserves are classified as compliance coal which meets the 1.2 lb SO 2 /mm Btu standard of Phase II of the Clean Air Act. Our compliance coal reserves are located in Pennsylvania, Wyoming, West Virginia, and Virginia/Kentucky.

As with most coal-producing companies that operate in Appalachia, which include our operations in Pennsylvania, West Virginia, and Virginia/Kentucky, the great majority of our Appalachian reserves are subject to leases from third-party landowners. These leases convey mining rights to the coal producer in exchange for a percentage of gross sales in the form of a royalty payment to the lessor, subject to minimum payments. Of our Appalachian reserve holdings at December 31, 2010, 410.9 million tons of reserves were owned and required no royalty or per-ton payment to other parties. Our remaining Appalachian reserve holdings at December 31, 2010, of 1,160.9 million tons were leased and require minimum royalty and/or per-ton payments.

Our mines in Wyoming are subject to federal coal leases that are administered by the U.S. Department of Interior under the Federal Coal Leasing Amendment Act of 1976. Each lease requires diligent development of the lease within ten years of the lease award with a required coal extraction of 1.0% of the reserves within that 10-year period. At the end of the 10-year development period, the mines are required to maintain continuous operations, as defined in the applicable leasing regulations. All of our federal leases are in full compliance with these regulations. We pay to the federal government an annual rent of $3.00 per acre and production royalties of 12.5% of gross proceeds on surface mined coal. Effective October 1, 2008, the Federal Government remits 48% of royalties, rentals and any lease bonus payments to the state of Wyoming. Of our Wyoming reserve holdings at December 31, 2010, 23.5 million tons of reserves are owned and require no royalty or per-ton payments. Our remaining Wyoming reserve holdings at December 31, 2010, of 629.7 million tons were leased and were subject to the terms described above.

Our idled mine in Illinois (“Wabash”) is subject to coal leases and requires payments of minimum royalties, payable in periodic installments. We expect to continue leasing these reserves until future development is feasible. Our reserve holdings attributable to Wabash at December 31, 2010 were 28.6 million tons.

Although our coal leases have varying renewal terms and conditions, they generally last for the economic life of the reserves. According to our current mine plans, any leased reserves assigned to a currently active operation will be mined during the tenure of the applicable lease. Because the great majority of our leased or

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owned properties and mineral rights are covered by detailed title abstracts prepared when the respective properties were acquired by predecessors in title to us and our current lessors, we generally do not thoroughly verify title to, or maintain title insurance policies on, our leased or owned properties and mineral rights.

The following table provides the “quality” (sulfur content and average Btu content per pound) of our total recoverable reserves as of December 31, 2010:

The following table summarizes, by regional business unit, the tonnage of our total recoverable reserves that is assigned to our operating mines, our property interest in those reserves and whether the reserves consist of steam or metallurgical coal, as of December 31, 2010:

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The following map shows the locations of Alpha’s properties as of December 31, 2010:

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The following map shows the locations of Alpha’s shipping points as of December 31, 2010:

See Item 1, “Business”, for additional information regarding our coal operations and properties.

Mine Safety and Health Administration Data

The information required under the Dodd-Frank Wall Street Reform and Consumer Protection Act of 2010 has been included in Exhibit 99.1 to this Annual Report on Form 10-K and incorporated by reference herein.

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SIGNATURES

Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

Date: April 21, 2011

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