Exhibit 99.1

National Interstate Corporation Reports 2012 Second Quarter Results

Richfield, Ohio, July 31, 2012 – National Interstate Corporation (Nasdaq: NATL) today reported results for the 2012 second quarter and first six months. Net after-tax earnings from operations per share, diluted of $0.36 for the 2012 second quarter and $0.79 for the 2012 first six months decreased compared to the same prior year periods reflecting lower underwriting profits partially offset by higher net investment income. Gross premiums written of $160.1 million for the 2012 second quarter increased 2.3% and gross premiums written $290.4 million for the 2012 first six months were flat compared to the same periods last year.

Earnings

The table below shows the Company’s net income per share determined in accordance with U.S. generally accepted accounting principles (GAAP), reconciled to net after-tax earnings from operations, which is a non-GAAP financial measure to better reflect the results related to the ongoing business.

Net after-tax earnings from operations include underwriting income and net investment income. After-tax realized gains from investments and the after-tax impact on underwriting results related to the balance sheet guaranty from the Vanliner acquisition are separately presented to better reflect the results related to ongoing business.

Underwriting Results:

These underwriting ratios exclude the impact of the runoff of the guaranteed Vanliner business based on premiums earned of $0.1 million for the 2012 first six months and $8.3 million and $23.5 million for the 2011 second quarter and first six months, respectively. There were no such premiums earned in the 2012 second quarter.

Dave Michelson, President and Chief Executive Officer said, “We were disappointed with the underwriting results for the 2012 second quarter which contributed to an elevated combined ratio for the first half of 2012. We have experienced severity in several of our commercial products and our specialty personal lines products are not improving as quickly as we would like. The combination of timing of larger claims, loss cost inflation, and the delayed impact of the rate increases we are now getting in most products has contributed to the rise in our loss and loss adjustment expense (LAE) ratio. On the positive side, our underwriting expenses are well controlled and have remained flat.”

Claims: The loss and LAE ratio for the 2012 second quarter of 74.7% was 2.5 percentage points higher than the 2011 second quarter, which contributed to the 4.2 percentage point variance when comparing the first six months of 2012 and 2011. In both the 2012 first and second quarters, the Company has experienced higher than average claims, primarily severity, in several commercial products which has contributed to the higher than expected loss and LAE ratios. The Company manages a significant number of unique commercial

products and those impacted by the higher claims have historically been profitable. In addition, claims related to the specialty personal lines products remain elevated in both 2012 and 2011.

For the 2012 second quarter the Company had favorable development from prior year claims of $0.2 million which compares to $0.7 million of favorable development for the 2011 second quarter.

Underwriting Expenses: The underwriting expense ratios for the second quarter and first six months of both years are consistent and within the expected range.

Investments:

Net investment income of $9.0 million for the 2012 second quarter and $18.1 million for the 2012 first six months was 14.8% and 23.4% above the same periods last year, respectively. The higher investment income is attributable to the portfolio repositioning that occurred in the last half of 2011 when the Company took advantage of the steep yield curve and volatility in the fixed income sectors to reposition its portfolio into higher yielding investments. As expected, 2012 second quarter net investment income was flat compared to the 2012 first quarter given the continued low fixed income yields that are available for maturing investments and cash flows from operations.

The Company maintains a high quality and diversified portfolio with approximately 94% of its fixed income portfolio rated NAIC 1 or 2 and an effective duration of 4.0 years. The fair value and unrealized gains (losses) of fixed maturities and equity securities were as follows:

Gross Premiums Written

The table below summarizes gross premiums written by business component:

Alternative Risk Transfer (ART): The ART component grew 3.9% for the 2012 second quarter and was slightly down for the 2012 first six months compared to the same 2011 periods. The growth in this component was attributable to new customers, an increase in exposures and the continuing very high customer retention. Offsetting the growth was the residual impact of a product that was terminated in 2011.

Transportation: The Transportation component grew approximately 3.0% in the 2012 second quarter and first six months due to an increase in exposures and new customers in the truck and moving and storage products. In addition, a portion of the growth is the result of rate increases that are starting to occur with some regularity.

Specialty Personal Lines: Gross premiums written in the specialty personal lines component again declined in the 2012 second quarter, resulting in a decrease of 8.7% for the 2012 first six months. Underwriting and pricing actions related to the commercial vehicle product and a trend towards purchasing direct versus through agents for the recreational vehicle product have adversely affected the top line for this component.

Hawaii and Alaska: Gross premiums written for the 2012 second quarter and first six months were 4.3% and 4.8% percent ahead of the same periods last year, respectively. For the past several quarters, Hawaii and Alaska gross premiums written have varied between single digit growth and single digit decline.

Mr. Michelson commented, “We expected flat top line results through the first half of 2012 because of the actions we took last year related to two products in the program business portion of the ART component, which comprised 8.2% of 2011 second quarter gross premiums written, and underwriting actions related to the specialty personal lines products. However, we are encouraged by the modest overall increase in gross premiums written for the quarter and growth in three of the four components.”

Summary Comments

“Our top line is performing as expected and improved investment income has added to our profitability,” stated Mr. Michelson. “However, our underwriting results for the first six months of 2012 are below our expectations of a combined ratio in the low to mid 90’s. Adverse claims including large claims in traditionally well performing products have impacted the results. We have and will continue to closely monitor all large commercial accounts not meeting our underwriting expectations for appropriate action, and we are getting rate increases in most of our commercial products. In our Specialty Personal Lines component, we have filed rate increases in virtually all states in both the recreation vehicle and commercial vehicle products. We believe all of these actions will have a favorable impact going forward.”

Earnings Conference Call

The Company will hold a conference call to discuss the 2012 second quarter results tomorrow, Wednesday, August 1, 2012 at 10:00 a.m. Eastern 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 (877) 837-3911. Please dial in 5 to 10 minutes prior to the scheduled starting time. 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

An Insurance Experience Built Around You.

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@natl.com

www.natl.com

NATIONAL INTERSTATE CORPORATION

SELECTED FINANCIAL DATA

(In thousands, except per share data)