Loss aversion, the principle that losses loom larger than gains, is among the most widely accepted ideas in the social sciences. Behavioral finance is a division of finance science that investigates the behavior of the financial markets regarding their psychology. Loss aversion is among the behavioral effects of the investors that have bad consequences and leads to a decrease in the portfolio return and makes the potential of receiving benefit limited. considering the salience of the role of the investing companies in selecting the optimal investment portfolio in the stock exchange of Tehran. Investors unfamiliar with financial markets and investor retailers have reduced their risk by turning to investment firms and expecting them to attract the best possible returns on their investments. The current research has investigated the disposition effect of loss aversion in the portfolio of 50 investment companies that includes 1100 companies for 3 succeeding years from 2017 to 2019 monthly. In order to categorize the data and define the variables and analyze them, two software has been used "Eviews" and "Excel". The results of the research confirm that there exists a disposition effect of the loss aversion in the investment companies of the stock exchange of Tehran. پرونده مقاله