The cost of capital is one of the most important and key tools in many financial and management decisions that are affected by several factors. Corporate governance and management behaviors are among the most important of these factors, also considering the ability to attract capital and provide financing. Therefore, the purpose of this study is to examine the relationship between financial flexibility and agency costs with stock diversification. The statistical population of the study is companies listed on the Tehran Stock Exchange and the sample under study includes 165 companies listed during the years 2010 to 2024. The research method is descriptive and in terms of the relationship between variables, it is a combination of causality and it is applied in terms of purpose. The regression method and panel data as well as the fixed effects model were used to process and test the hypotheses. The results obtained from the analysis of the hypotheses showed that political costs have a positive and significant relationship with the cost of capital and managerial myopia has a positive and significant relationship with the cost of capital.