Funding was in trouble. Despite such crises, the issue of corporate finance has become one of the most important issues in the financial literature. The turbulent and changing economic environment of companies, which is characterized by the globalization of markets, changing customer needs and increasing competition in the product market, has forced companies to constantly seek to improve their performance. The purpose of this study is to study the effect of financial constraints on the relationship between investment opportunities and financing costs. The statistical population of the research is the companies listed on the Tehran Stock Exchange and the study sample includes 133 companies listed during the years 2016 to 2020. The research method is descriptive and in terms of the relationship between variables is a combination of correlation and is applied in terms of purpose. To process and test the hypotheses, regression method and panel data as well as fixed effects model have been used. The results of the analysis of hypotheses showed that investment opportunities have a positive and significant relationship with financing costs and financial constraints have a positive and significant effect on the relationship between investment opportunities and financing costs. According to the research results, it is suggested to financial information analysts and capital market activists that in investment decisions and stock market evaluation, special attention be paid to the cost of financing companies as well as the ratio of market value to book value of equity and its effect on stock returns. Have.