Abstract: An experiment was conducted to estimate the optimal dietary zinc (Zn) level for broiler chicks fed a corn-soybean meal diet. A total of 384 day-old male broiler chicks were assigned randomly into dietary treatments for 21 days These treatments included a basal corn-soybean meal diet (28.37 mg Zn /kg diet) and the diets supplemented with 0, 20, 40, 60, 80, 100, 120 or 140 mg/kg of Zn in the form of reagent grade ZnSO₄·7H₂O. All treatments were replicated 6 times using 8 chicks per pen. Tissue Zn concentration and Zn metalloenzyme activity were analyzed for choosing suitable criterion to determine the optimal dietary Zn level for broilers. Regression analysis was performed to estimate optimal dietary Zn level in the presence of quadratic or asymptotic responses. Results showed that daily weight gain and daily feed intake were increased with dietary Zn level increased (P<0.05), and the maximum daily weight gain and daily feed intake were observed in the diet supplemented with 20 and 40 mg/kg of Zn (48.37 and 68.37 mg/kg total dietary Zn). According to asymptotic model, the optimal Zn requirement of chicks from hatch to 21 d of age was 30.78 mg/kg for pancreas Zn and 3386 mg/kg for bone Zn, respectively. Quadratic responses were exhibited by serum 5′-nucleotidase (5′-NT) activity, resulting in total optimal dietary levels of 52.13 mg/kg. Based on the results, the optimal dietary Zn level of chicks from hatch to 21 d of age is 80 mg/kg.