Abstract: This study has been done with the aim of evaluate appointment of its relationship and size and limit between two or more quantitative variables (firm size, industry type, profitability). Methodology is type of correlation that the purpose of potential correlation study between these variables is establishing a relationship, or lack of that and applying the relationships in doing forecasts. Statistical population includes companies listed on the Tehran Stock Exchange in the five industries "vehicle and manufacturing the parts", "pharmaceutical materials and products", "investments", "basic metals" and "electronic machineries" in time interval of 2008-2014. During the period of financial information relating to 35 companies in 2008-2014 were evaluated that were selected by using relative classification sampling. Statistical data show that capital structure is not the same in different industries and the amount of assets and profitability of companies is associated with type of capital structure but the rate of sale has no effect on capital structure. In the next step through multiple regression effect these factors was investigated once on the capital structure which shows that internal factors of the companies has effect on capital structure and there is a relationship between them. So, financial managers must consider their domestic factors during financing in order to achieve an optimal capital structure to increase the value of their company.