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This paper investigates the impact of Audit Opinion and Audit Opinion Improvement on firms’ stock price and trade volumes in Tehran Stock Exchange (TSE) It is assumed that Audit Opinion and Audit Opinion Improvement reflex good and bad news about firms. The sample consists of 75 firms listed in TSE. The Statistic Method is Pooled Data Regression. In order to quantify Audit Opinion Improvement, Li and Wu (2004) method was employed. The results show that there is a positive and significant relationship between Audit Opinion and firms’ stock prices and trade volumes. Audit Opinion Improvement is also positively correlated with stock prices and trade volumes. Results show that auditors’ reports have information content in TSE and investors consider its type and improvement in their decision making.