This study investigated the financial performances of environmental policy by using the long‐term effect of ISO 14001 certification. Drawing on a natural‐resource‐based view of the firm, this study examined abnormal performances of ISO 14001 certified firms on the New York Stock Exchange (NYSE) and National Association of Securities Dealers Automated Quotations (NASDAQ) in the USA during the period 1996–2010 employing a rigorous event study methodology. The results indicate that the profitability variables showed immediate positive abnormal effects after firms applied for the ISO 14001 certification, while the market benefit variable showed gradual improvements after obtaining the certification. However, ISO 14001 did not show significant effect on the internal process improvement. Overall, ISO 14001 was found beneficial to the firm in the long run from the perspectives of profitability and market benefits. Copyright © 2017 John Wiley & Sons, Ltd and ERP Environment