An experiment was conducted to evaluate the effect of three levels of CP (17.70, 20.80, 23.97) with constant essential amino acids on performance, blood parameters and nutrients retention of broiler chickens during starter period at 0-21 days of age. A completely randomized design with 3 dietary treatments and four replicates per treatment (10 birds per pen) was performed. Corn-soybean meal based diets fed to broiler chickens to provide 2900 kcal/kg ME. Chromium oxide was used at 18 days of age for measuring nutrient retention. Feed and water were provided ad libitum throughout the experiment. The average body weight gain (BWG), feed intake and water intake were recorded weekly. Blood samples were obtained at 21 days of age in heparinised tubes for measuring blood constituents. At this day, chickens from each replicate of treatment were randomly selected and tibia bone samples were taken for mineral retention study. Results of this experiment showed that CP content had no significant effect on feed intake (weekly and overall periods). Similar trend was observed for water intake except for 1-21 days, which 17.70% CP significantly decreased water intake (P < 0.05). Body weight gain (BWG) significantly reduced and FCR increased (P < 0.05) with decreasing CP. A change in CP content of the diets had no significant effect on blood parameters except for sodium (Na+), potassium (K+) and glucose (Glu) concentration. Increasing CP content of the diet significantly decreased P, Ca and N retention (P < 0.001). Increasing the CP content of the diet led to a decrease in length of tibia bone. Reducing the CP content of the diet, significantly decreased tibia bone P.

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