Angiotensin I-converting enzyme (ACE) inhibitory peptides have been shown to have antihypertensive effects and have been utilized for physiologically functional foods and pharmaceuticals. The ACE inhibitory ability of a hydrolysate is determined by its peptide composition. However, the peptide composition of a hydrolysate depends on proteolytic enzyme and the hydrolysis conditions. In this study, the effect of process conditions on the ACE inhibitory activity of rice dregs hydrolyzed with a trypsin was investigated systematically using response surface methodology. It was shown that the ACE inhibitory activity of rice dregs hydrolysates could be controlled by regulation of five process conditions. Hydrolysis conditions for optimal ACE inhibition were defined using the response surface model of fractional factorial design (FFD), steepest ascent design, and central composite design (CCD).