The aim of the study is to examine the effect of ownership concentration by sum of major shareholders on the firm value. The sample used for the research included 69 firms listed during the years 2005-2009 in Tehran Stock exchange. The statistical method used for testing the research hypotheses was "Panel Data". To test the relationship between ownership of major stockholders and firm value, multi-linear and nonlinear regression was utilized. The findings are indicative of the existence of a significant and positive linear relationship between ownership concentration of sum of major stockholders and firm value and also of lack of nonlinear relationship between the square sum of ownership by major stockholders and firm value. The results of piecewise test of sum of major stockholders shows that there is no significant relationship between sum of major stockholders and firm value in the break points of 10% and 25% and 50% and 70%.
