Silkworm pupae protein is a good source of high quality protein. The hydrolyzates of silkworm pupae protein catalyzed by neutrase, pepsin, acidic protease (Asperqiius usamii NO. 537), flavourzyme, alcalase, and trypsin with inhibitory activity on angiotensin I-converting enzyme (ACE) were identified by HPLC. The hydrolyzates catalyzed by acidic protease exerted the highest inhibitory activity on ACE. The hydrolyzing conditions were optimized by one-factor, factional factorial (FFD), and center composite (CCD) design methods, and response surface methodology (RSM). Statistical analyses showed that regression of the second-order model equation is suitable to describe ACE inhibitory bioactivity. The predicted inhibitory activity of hydrolyzates on ACE was 73.5 % at a concentration of 2.0 mg/ml. Optimized RSM technique decreased IC(50) of hydrolyzates inhibiting ACE to 1.4 mg/mL from 2.5 mg/ml. The molecular weight of the components of the hydrolyzates with inhibitory activity on ACE varied from less than 500 to about 1000 Da by ultra-filter analysis. These studies suggest that hydrolyzates of silkworm protein contain ACE inhibitory activity that could form a potential source of ACE inhibitor drugs.