Earning is considered as one of the most basic and important items of financial statements in financial and economic decisions, which is considered by many people. But sometimes managers, due to maximizing their own interests, may report earning in a way that contradicts the goal of providing the general interests of users. Earning management refers to the set of actions that managers take in order to decorate information to achieve specific goals. One of the factors affecting earning management is financial reporting quality and internal audit. Based on the mentioned materials, the main goal of this research is to investigate the effect of internal audit on the relationship between financial reporting quality and earning management in companies listed in the Tehran Stock Exchange. In order to achieve the above objectives, two hypotheses were formulated. Therefore, in order to test the research hypothesis, a sample of 169 firms was selected from the firms listed in Tehran Stock Exchange during the period 2015 to 2023. Multivariate regression model based on combined data was used to test the research hypothesis. The findings of the research showed that financial reporting quality has a negative and significant effect on the earnings management, that is, an increase in financial reporting quality leads to a decrease in earnings management; In addition, the results indicate that internal audit strengthens the relationship between financial reporting quality and earnings management.