Non-transparency of financial information is defined as the deliberate denial of access to information, the misrepresentation of information, or the inability of the market to ensure the adequacy of relevance and quality of information. After the scandals of the beginning of the third millennium and the emergence of financial crises in the world's stock markets, the issue of information transparency in the financial reporting process has received widespread attention. Therefore, the purpose of this research is to study the effect of auditor's reputation on the relationship between active institutional ownership and the lack of transparency of financial reporting. The statistical research population of companies admitted to the Tehran Stock Exchange and the studied sample includes 152 companies admitted during the years 2017 to 2018. The research method is descriptive and causal-correlation in terms of the relationship between the variables, and it is practical in terms of the goal. The regression method and panel data as well as the fixed effects model were used to process and test the hypotheses. The results obtained from the analysis of the hypotheses showed that active institutional ownership has a negative and significant relationship with the lack of transparency of financial reporting, and the auditor's reputation has a positive and significant effect on the relationship between active institutional ownership and the lack of transparency of financial reporting at the 5% error level.