The conflict between investors' interests and managers is an important issue which has been attracted researchers attentions in last decade. According to the related literature, corporate governance body of research has offered alternatives for solving this problem such as Ownership structure. This research investigates the effect of ownership structure on firm performance. The factors which identified ownership structure were the number of block holders and kind of ownership were studied. Meanwhile, industry factor was applied in the research model. The samples include 56 firms whose shares' transactions were active in Iran security exchange market from 1999 to 2004. The firms were classified according to their kind of ownership which involve private, semiprivate, public, semipublic and institutions (NGOs). By applying multiple correlation and analysis of variance statistical tests, it was found that ROE ratio is affected by the industry factor ,but the difference of P/E ratio between studied industries was not significant.We have found when the number of block holders increase, ROE ratio also increase. This finding can be explained for strong control by large shareholders and enforcing their synergy in decision making. According to the sample size, some the relationships between ownership structure factors and firm performance was not significant. Other results show that the large equity ownership by a mix of private and public sectors in firm ownership structure improve return on equities firm due to the role of government in governing and controlling the firms. Finally, the results show that performance of corporations in Iran security exchange was affected mostly by industry and economical circumstances rather than managerial decisions and ownership structure.