Abstarct: Abstract This study, examine the impact of past portfolio performance on the future decisions of investors with the mediating role of behavioral biases in the Tehran Stock Exchange. Thus the past performance of the individual as an independent variable and psychological factors including optimism, overconfidence, degree of risk-taking as dependent variables and And used the structural equation method to examine this relevance. Also, a questionnaire was prepared for this purpose and by examining its validity and reliability, a statistical sample of 382 people was used by Cochran sampling method. The results show that the past performance of the portfolio will increase by 0.32 points in optimism, as well as 0.28 in overconfidence, 0.28 in risk-taking and 0.35 in conservatism. Observations also indicate that the increase in risk-taking and conservatism lead to high portfolio turnover and increases the volume of transactions and risk-taking, therefore, for this purpose, it is recommended that, investors' information about behavioral biases be increased so that investors, so that investors have the least decision error.