The study was to purify cathepsin B from horse mackerel (Trachurus japonicus) and fractionate the angiotensin I-converting enzyme (ACE) inhibitory peptides from catheptic hydrolysates. The purification fold, yield and molecular weight of horse mackerel cathepsin B were 7,814 fold, 3.8% and 27,500Da respectively. Four fractions with ACE-inhibitory activity appeared on Shephadex G-25 chromatography. Their molecular weight were 1,700, 1,400, 900and 550Da respectively. The IC50 of four hydrolysates were 592.6, 190.3, 91.6 and 65.8μg peptide/ml. The results suggest that horse mackerel protein hydrolysates by cathepsin B might be a potential ingredient for the formulation of hypotensive functional foods.