Abstarct: This paper investigates the relationship between ownership structure and firm performance using a sample of Iranian manufacturing and trading firms. We employ Ordinary Least Squares (OLS) methods to empirically examine relationship between ownership structure and performance. Using these performance measures we examine if more efficient firms have more or less ownership concentration in their ownership structure. We summarize the effects of ownership concentration on performance in terms of two hypotheses: 1. more concentrated ownership should have a positive effect on firm performance. 2. This relationship is significantly different in manufacturing and trading firms. Using quantile regressions we test the effect of ownership concentration on performance and thus the empirical validity of the two hypotheses across different ownership structure choices. We also test the direct relationship from ownership concentration to efficiency stipulated by the Jensen and Meckling (1976) agency cost model