The purpose of this study was the purification and characterization of an angiotensin I converting enzyme (ACE) inhibitory peptide purified from enzymatic hydrolysates of rainbow trout Oncorhynchus mykiss muscle. After removal of lipid, the approximate composition analysis of the rainbow trout revealed 24.4%, 1.7%, and 68.3% for protein, lipid, and moisture, respectively. Among six hydrolysates, the peptic hydrolysate exhibited the highest ACE inhibitory activity. We attempted to purify ACE inhibitory peptides from peptic hydrolysate using high performance liquid chromatography on an ODS column. The $IC_{50}$ value of purified ACE inhibitory peptide was $63.9{mu}M$. The amino acid sequence of the peptide was identified as Lys-Val-Asn-Gly-Pro-Ala-Met-Ser-Pro-Asn-Ala-Asn, with a molecular weight of 1,220 Da, and the Lineweaver-Burk plots suggested that they act as a competitive inhibitor against ACE. Our study suggested that novel ACE inhibitory peptides purified from rainbow trout muscle protein may be beneficial as anti-hypertension compounds in functional foods.