en عمومى General پژوهشي Research The paper efforts to analyze the determinants and their relationship with the capital structure of the listed companies according to the trade-off (TOT) and pecking order (POT) theories. The investigation has used panel data of five variables from Iranian listed companies for a period of ten years. The data consists of 1230 observations regarding stock market and accounting data covering the period from 2001-2010. The inferences are drawn using trade-off theory and Pecking order theory. OLS and GMM analysis is applied for hypothesis testing to determine the influence of the predictor variable. The variables of tax, profit, size, growth and capital intensity factors are included to represent the potential influence of traditional theories. The study analyzes the impact of the financial factors on the debt and equity structure of the Iranian firms. The results indicate that the size, tax and capital intensity are positively related to the capital structure. In addition, profitability is negatively related to the capital structure. However, growth is insignificant. The results of the firm size, tax and capital intensity are consistent with the trade-off theory and the result of the profitability is consistent with the pecking order theory. Capital Structure, Trade-Off Theory, Pecking Order Theory, Capital Intensity. 45 55 http://ijorlu.lahijan.iau.ir/browse.php?a_code=A-10-448-1&slc_lang=en&sid=1 A. Ahmadimousaabad aiyuob151@yahoo.com 10031947532846001207 10031947532846001207 Yes Department of Management, Lahijan Branch, Islamic Azad University, Lahijan, Iran F. Mohammadi Nodeh mnfazel2@gmail.com 10031947532846001208 10031947532846001208 No Department of Humanities, Lahijan Branch, Islamic Azad University, Lahijan, Iran