Competition in the product market can increase or decrease agency problems and distort financial reporting; It can also have an effect on managers' motivation to manipulate earnings or manage reported earnings. The purpose of this study is to investigate the effect of managerial ownership on the relationship between customer concentration and earning management. The statistical population of the study is all companies listed on the Tehran Stock Exchange and using the systematic elimination sampling method, 142 companies were selected as the sample of the research in an 8-year period between 2012 and 2020. The method used to collect information is a library and the relevant data for measuring variables were collected from Codal website and companies' financial statements and then Eviews software was used to test the research hypotheses. The results of testing the research hypotheses showed that there is a negative and significant relationship between customer Concentration and Earning management and management ownership affects the relationship between customer Concentration and Earning management.