UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of report (Date of earliest event reported): August 1, 2013

THE FRONTIER FUND

FRONTIER DIVERSIFIED SERIES;

FRONTIER LONG/SHORT COMMODITY SERIES;

BALANCED SERIES

(Exact Name of Registrant as Specified in Charter)

c/o Equinox Fund Management, LLC

1775 Sherman Street, Suite 2500

Denver, Colorado 80203

(Address of Principal Executive Offices)

(303) 837-0600

(Registrant’s telephone number, including area code)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Item 1.01. Entry into a Material Definitive Agreement.

Effective August 1, 2013, Emil Van Essen, LLC is elevated to a major advisor for the Frontier Diversified Series and Crabel Capital Management, LLC, FORT LP and Quest Partners LLC are added to the portfolio of Frontier Diversified Series as major advisors. The advisory agreements with respect to the arrangements with Emil Van Essen, FORT LP and Quest Partners LLC are substantially similar to the Form of Advisory Agreement, included as Exhibit 10.3 to the Registration Statement on Form S-1 filed by The Frontier Fund on February 2, 2010 and incorporated herein by reference. The Frontier Diversified Series will gain exposure to the trading program of Crabel Capital Management, LLC via a total return swap with Deutsche Bank AG London (the “DB Diversified Swap”). The term of the DB Diversified Swap is five years.

Effective August 1, 2013, Emil Van Essen, LLC is elevated to a major advisor for the Frontier Long/Short Commodity Series and J E Moody & Company, Abraham Trading Company, Krom River Trading AG and Commodity Strategies AG are added to the portfolio of Frontier Long/Short Commodity Series as major advisors. The advisory agreement with respect to the arrangement with Emil Van Essen is substantially similar to the Form of Advisory Agreement, included as Exhibit 10.3 to the Registration Statement on Form S-1 filed by The Frontier Fund on February 2, 2010 and incorporated herein by reference. The Frontier Long/Short Commodity Series will gain exposure to the trading program of J E Moody & Company, Abraham Trading Company, Krom River Trading AG and Commodity Strategies AG via a total return swap with Deutsche Bank AG London (the “DB Commodity Long/Short Swap”). The term of the DB Commodity Long/Short Swap is five years.

Effective August 1, 2013, BH-DG Systematic Trading LLP, FORT LP and QuantMetrics Capital Management LLP are elevated to major advisors for the Balanced Series of the Frontier Fund and Campbell & Company, Inc. and Quantica Capital are added to the portfolio of Balanced Series as major advisors. The advisory agreements with respect to the arrangements with FORT LP and QuantMetrics Capital Management LLP are substantially similar to the Form of Advisory Agreement, included as Exhibit 10.3 to the Registration Statement on Form S-1 filed by The Frontier Fund on February 2, 2010 and incorporated herein by reference. The Balanced Series gains exposure to the trading program of BH-DG Systematic Trading LLP, Campbell & Company, Inc. and Quantica Capital via total return swaps with Deutsche Bank AG London (the “DB Swaps”). The term of each of the DB Swaps is five years.

Item 9.01. Financial Statements and Exhibits.

(d) The following exhibit is incorporated by reference:

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SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

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SIGNATURES

Pursuant to the requirements 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.

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SIGNATURES

Pursuant to the requirements 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.

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SIGNATURES

Pursuant to the requirements 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.

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