Given the tax advantages, costs of financial distress and bankruptcy, agency theory and other benefits and costs, the capital structure of the company will affect its value. In addition, company size affects many aspects of a business operation. The purpose of the present study is to investigate the effect of firm size on the relationship between capital structure and financial bankruptcy in listed companies in Tehran Stock Exchange. This study is applied in terms of purpose, quantitative data and descriptive correlational in terms of data collection. And it is one of the positive accounting research. This research will be conducted within the framework of inductive reasoning. It is also the type of runtime after the event (past information). There is a significant relationship between the degree of financial leverage and the financial bankruptcy of companies listed in Tehran Stock Exchange. There is a significant relationship between the long-term debt ratio and the financial bankruptcy of the companies listed on the Tehran Stock Exchange. There is a significant relationship between the short-term debt ratio and the financial bankruptcy of the companies listed on the Tehran Stock Exchange. The size of the company has no significant effect on the relationship between the degree of financial leverage and the financial bankruptcy of the companies listed on the Tehran Stock Exchange. The size of the firm has a significant effect on the relationship between long-term debt ratios and financial bankruptcy of companies listed on the Tehran Stock Exchange.