Abstract: The meat of hard clam was extracted using hot water. The residual meat was freeze-dried then hydrolyzed at 50 °C for 5 h by Protamex (PX). The inhibitory effects of hot water extract and hydrolysate against angiotensin I converting enzyme (ACE) were investigated. The IC50 value of hot water extract and hydrolysate on ACE were 1.090 and 0.036 mg/ml, respectively. The PX hydrolysate was separated into five fractions by size exclusion chromatography on a Sephadex G-25 column. The fifth fraction of the hydrolysate having molecular weight ranged 350–300 Da showed the highest inhibitory efficiency ratio (IER) being 5831%/(mg ml). The amino acid sequence of the inhibitory peptide was Tyr-Asn (IC50 = 51 μM). The hydrolysate showed mixed-type inhibition kinetics while Captopril, the positive control, showed competitive inhibition on ACE. Their Ki values were 0.027 mg/ml and 0.0067 μg/ml, respectively.