The main purpose of the development and implementation of corporate governance principles is encourage managers to take steps towards the interests of the company, instead of pursuing personal interests and build confidence among financial market participants. So we can expect that proper development and implementation of the above principles increases the confidence of owners and other interest groups, and ultimately reduces regulatory and agency costs. This study is carried out with the aim of determining the direction and extent of effectiveness of each mechanism of ownership structure corporate governance on agency costs. This study is a causal-post-eventual research in terms of method, and considering its application in capital market is an applied research, in terms of objective. Also test of variables in this study is multivariate regression of panel data. Overall, the results of data analysis of 124 companies listed in Tehran Stock Exchange during the years 1382-1389 shows that the percentage of free floats have a positive effect on agency costs, and the percentage of state ownership, the percentage of directorate ownership and institutional ownership has no effect on agency cost. [Y.B. Nhandi, H. Baradaran hasanzade, GH. Sharifzade. The effect of ownership structure of corporate governance on agency cost. Life Sci J 2012;9(4):3002-3010]. (ISSN: 1097-8135). http://www.lifesciencesite.com. 441