Exhibit 99.1

National Interstate Corporation Reports 2011 Second Quarter Results

Richfield, Ohio, August 3, 2011 – National Interstate Corporation (Nasdaq: NATL) today reported results for the 2011 second quarter and first six months. Gross premiums written of $156.5 million for the 2011 second quarter and $290.8 million for the 2011 first six months both increased due to the favorable impact from the acquisition of Vanliner Insurance Company in July 2010 as well as growth in existing businesses. Net after-tax earnings from operations per share, diluted of $.40 and $.92 for the 2011 second quarter and first six months, respectively, also increased compared to the same periods last year reflecting higher net investment income offset by lower underwriting profits.

The table below shows the Company’s net income determined in accordance with U.S. generally accepted accounting principles (GAAP), reconciled between net after-tax earnings from operations, after-tax net realized gain from investments, the after-tax impact from the balance sheet guaranty related to the Vanliner acquisition, and the change in the valuation allowance on deferred tax assets related to net capital losses, all of which are non-GAAP financial measures.

Net after-tax earnings from operations are comprised of underwriting income and net investment income. The Company separately reports after-tax realized gain from investments and the after-tax impact on underwriting results related to the balance sheet guaranty from the Vanliner acquisition.

Underwriting Results:

The following ratios exclude the impact from the balance sheet guaranty associated with the Vanliner acquisition to reflect the results of ongoing underwriting operations:

The Company experienced higher combined ratios for the 2011 second quarter and first six months when compared to the same 2010 periods due to higher claims costs that were partially offset by lower underwriting expenses.

Claims: The loss and loss adjustment expense (LAE) ratios for the 2011 second quarter and first six months were both approximately 6 percentage points higher than the same periods last year. The Company has experienced several periods of single digit rate decreases in the traditional transportation products that have contributed to rising loss and LAE ratios. In addition, the period over period variances reflect the 2011 impact of elevated losses in the specialty personal lines component.

The Company experienced favorable development from prior year claims, excluding the Vanliner balance sheet guaranty, of $0.7 million which reduced the loss and LAE ratio by 0.7 percentage points for the 2011 second quarter, compared to 2.2 percentage points of favorable development for the 2010 second quarter.

Expenses: The Company’s quarterly underwriting expense ratios could vary based on the mix of business written or non-recurring items such as costs associated with the Vanliner acquisition, but the Company has consistently reported underwriting expense ratios in the mid 20% range for the past several years. The underwriting expense ratio of 23.4% for the 2011 second quarter and first six months is within the expected range and approximately 2 percentage points better than the same 2010 periods in part due to leveraging fixed expenses over a larger earned premium base.

Dave Michelson, President and Chief Executive Officer stated, “As we expected, our loss and LAE ratios began trending up in the last half of 2010 when compared to the favorable claims results that we had been experiencing. Loss and LAE ratios in the 2011 second quarter were about 3 percentage points higher than the average for the prior three quarters, in part related to our personal lines component including seasonal losses in the recreational vehicle product. We have not identified any material areas of concern related to our claims costs and underwriting results for the first half of 2011 are in line with our expectations.”

Investments:

Investment Income: Net investment income of $7.8 million for the 2011 second quarter was 56% above the 2010 second quarter primarily due to the addition of the Vanliner portfolio. Investment income has increased gradually each quarter since adding the Vanliner portfolio and grew 13% from the 2011 first quarter as the cash and maturities from the acquired portfolio have been reinvested in higher yielding instruments.

The Company had net realized gain from investments of $1.3 million and $2.5 million for the 2011 second quarter and first six months, respectively, primarily from security sales at gains for reinvestment in comparable or higher yielding sectors. The realized gains in the first half of 2010 were from sales to generate funds for the Vanliner acquisition.

The Company maintains a high quality and diversified portfolio with approximately 90% of its cash and invested assets rated as investment grade and an average rating of AA- in its fixed income portfolio. The fair value and unrealized gains (losses) of fixed maturities and equity securities were as follows:

Impact from the Vanliner Guaranteed Runoff Business and Deferred Income Tax Valuation Allowance:

As previously disclosed, the seller of Vanliner provided National Interstate with comprehensive financial guarantees related to the runoff of Vanliner’s final closing balance sheet whereby both favorable and unfavorable balance sheet development inures to the seller. Additionally, as a result of purchase accounting requirements the Company was required to determine the fair value of the future economic benefit of the financial guarantees and acquired loss reserves, as of the date of acquisition, despite the fact that actual gains and losses related to the financial guaranty will be reflected in operations as they are incurred in future periods. As a result, the timing of the revenues and expenses associated with the guaranteed runoff business will not occur in the same period and will result in combined ratios which are not consistent with the negotiated combined ratio which was to approximate 100% for the Vanliner guaranteed business. As such, the after-tax impact from the runoff business guaranteed by the seller of $0.5 million and $1.7 million for the 2011 second quarter and first six months respectively, have been removed from the net after-tax earnings from operations to reflect only those results of the ongoing business.

In 2008, the Company established a valuation allowance on deferred tax assets associated with its net realized investment losses, primarily impairment charges during the financial crisis, which increased the 2008 effective tax rate. Beginning in the 2009 first quarter and continuing through the 2010 first quarter, this valuation allowance decreased resulting in a corresponding decrease in the federal income tax expense. No valuation allowance against deferred tax assets existed subsequent to March 31, 2010.

Gross Premiums Written:

Mr. Michelson commented, “Our top line had another strong quarter from both the addition of Vanliner business as well as growth in our Alternative Risk Transfer (ART) component. All other businesses are performing consistent with our expectations given the competitive pricing and other market conditions for our various niche insurance products.”

The table below summarizes gross premiums written by business component:

Alternative Risk Transfer (ART): The ART component grew 30% for the 2011 second quarter and 26% for the 2011 first six months compared to the same 2010 periods. The growth in this component was attributable to the continued above average renewal rate combined with the addition of new customers in existing programs. Also,

during the quarter, the Company introduced its second ART program for Vanliner’s moving and storage customers.

Transportation: Beginning in the 2010 third quarter, traditional business written as a result of the Vanliner acquisition was reported in the Transportation component. The growth in this component during the first half of this year was attributable to moving and storage premiums that have been written through Vanliner. Business being generated by the Company’s other transportation products in this component have been flat to down in 2011 when compared to 2010.

Specialty Personal Lines: Gross premiums written in the specialty personal lines component were down 15% for the 2011 second quarter, similar to the decline experienced in the 2011 first quarter. Underwriting and pricing actions related to the commercial vehicle product and a decline in quotes for the recreational vehicle product in 2011 have adversely affected the top line for this component.

Hawaii and Alaska: For the first time in several quarters, the Company experienced growth in this component. Market conditions remain competitive making it unclear whether growth can be sustained for the remainder of the year.

Summary Comments:

“We are pleased that through the first six months of 2011 both our gross premiums written and operating earnings are ahead of last year,” stated Mr. Michelson. “Our combined ratio of 93.1% is in the anticipated range, while investment income and realized gains are slightly ahead of expectations.”

Earnings Conference Call

The Company will hold a conference call to discuss the 2011 second quarter results tomorrow, Thursday, August 4, 2011 at 10:00 a.m. Eastern Daylight Saving Time. There are two communication modes available to listen to the call. Telephone access to the conference call and Q and A session will be available by dialing 888-713-4213 and providing the confirmation code 41865362. Please dial in 5 to 10 minutes prior to the scheduled starting time. To pre-register for the conference call, go to https://www.theconferencingservice.com/prereg/key.process?key=P4YHRR7PW and follow the instructions provided. The conference call will be broadcast live over the Internet. To listen to the call via the Internet, access our website at http://invest.natl.com and follow the instructions at the web cast link. The archived web cast will be available shortly after the call on our website

Forward-Looking Statements

This document, including any information incorporated by reference, contains “forward-looking statements” (within the meaning of the Private Securities Litigation Reform Act of 1995). All statements, trend analyses and other information contained in this press release relative to markets for our products and trends in our operations or financial results, as well as other statements including words such as “may,” “target,” “anticipate,” “believe,” “plan,” “estimate,” “expect,” “intend,” “project,” and other similar expressions, constitute forward-looking statements. We made these statements based on our plans and current analyses of our business and the insurance industry as a whole. We caution that these statements may and often do vary from actual results and the differences between these statements and actual results can be material. Factors that could contribute to these differences include, among other things: general economic conditions, any weaknesses in the financial markets and other factors, including prevailing interest rate levels and stock and credit market performance which may affect or continue to affect (among other things) our ability to sell our products and to collect amounts due to us, our ability to access capital resources and the costs associated with such access to capital and the market value of our investments; our ability to manage our growth strategy, customer response to new products and marketing initiatives; tax law and accounting changes; increasing competition in the sale of our insurance products and services and the retention of existing customers; changes in legal environment; regulatory changes or actions, including those relating to regulation of the sale, underwriting and pricing of insurance products and services and capital requirements; levels of natural catastrophes, terrorist events, incidents of war and other major losses; adequacy of

insurance reserves; and availability of reinsurance and ability of reinsurers to pay their obligations. The forward-looking statements herein are made only as of the date of this document. The Company assumes no obligation to publicly update any forward-looking statements.

About National Interstate Corporation

National Interstate Corporation (Nasdaq: NATL), founded in 1989, is the holding company for a specialty property-casualty insurance group which differentiates itself by offering products and services designed to meet the unique needs of niche markets. Products include insurance for passenger, truck, and moving and storage transportation companies, alternative risk transfer, or captive programs for commercial risks, specialty personal lines products focused primarily on recreational vehicle owners and small commercial vehicle accounts, and transportation and general commercial insurance in Hawaii and Alaska. The Company’s insurance subsidiaries, including the three primary insurers, National Interstate Insurance Company, Vanliner Insurance Company and Triumphe Casualty Company, are rated “A” (Excellent) by A.M. Best Company. Headquartered in Richfield, Ohio, National Interstate is an independently operated subsidiary of Great American Insurance Company, a property-casualty subsidiary of American Financial Group, Inc. (NYSE: AFG) (Nasdaq: AFG).

Contact:

Tanya Inama

National Interstate Corporation

877-837-0339

investorrelations@nationalinterstate.com

www.natl.com

NATIONAL INTERSTATE CORPORATION

SELECTED FINANCIAL DATA

(In thousands, except per share data)