Two hundred and forty one-day-old male broiler chickens (Ross-308) were fed corn-soybean basal diets with 0, 40, 80, or 160 mg/kg of an acid protease preparation from 1 to 42 days of age, in order to investigate the effects of an exogenous enzyme on growth performance, pancreatic trypsin activity and mRNA expression. Average daily gain (ADG) was greatest in the 160-mg/kg treatment group at 1-21 days, which was significantly higher than that in the control. After 42 days, average daily feed intake (ADFI) and ADG had significantly increased in the 80- and 160-mg/kg treatments (P < 0.05), with the 80-mg/kg treatment group having the highest ADFI and ADG values. The feed conversion ratio was not affected. Dietary acid protease at 80 and 160 mg/kg significantly decreased trypsin activity in the pancreas (relative to the control group) by 35.71 and 47.29%, respectively (P < 0.05). After 42 days, trypsin mRNA expression in the pancreas had significantly decreased by 19.5% in the 80-mg/kg treatment group relative to the control (P < 0.05). After 21 and 42 days, the diet supplemented with 160 mg/kg acid protease significantly decreased pancreatic trypsin mRNA by 19.6 and 37.7%, respectively, compared to the control. There were the 40-mg/kg treatment group and the control. Our results suggest that the amount of acid protease in the diet significantly affects trypsin activity and mRNA expression in broilers.