Due to studies on the effect of dietary protein on serum Growth Hormone (GH) and insulin-like growth factor-1 (IGF-1) concentrations in lambs fed at different levels of dietary protein are limited, this study was conducted to determine the effect of dietary protein level on GH and IGF-1 concentrations and performance in lambs. A total of 50 Kivircik male lambs were randomly selected for this experiment. Lambs were randomly assigned ten lambs to each of the five treatment diets containing 10, 12, 14, 16 and 18% CP to determine the effects of dietary CP level on performance, serum GH and IGF-1 concentrations. The initial body weight was similar for all lambs and averaged 26.20 +/- 1.10 kg. Final body weight of lambs were 37.31 +/- 1.46, 39.60 +/- 1.18, 39.85 +/- 1.24, 42.04 +/- 1.25 and 41.44 +/- 1.13 kg for groups fed 10, 12, 14, 16 and 18% CP, respectively and lambs fed 16 and 18% CP diet had significantly higher body weight than those of 10% CP (p<0.05). Mean concentrations of GH and IGF-1 in blood serum of lambs were not different significantly between groups.