Abstract Yak ( Bos grunniens) milk casein derived from Qula, a kind of acid curd cheese from northwestern China, was hydrolysed with alcalase. The hydrolysates collected at different hydrolysis times (0 min, 60 min, 120 min, 180 min, 240 min, 300 min, 360 min) were assayed for the inhibitory activity of angiotensin-I-converting enzyme (ACE), and the one obtained at 240 min hydrolysis showed the highest ACE inhibitory activity. The active hydrolysate was further consecutively separated by ultrafiltration with 10 kDa and then with 6 kDa molecular weight cut-off membranes into different parts, and the 6 kDa permeate showed the highest ACE-inhibiting activity. This active fraction was further purified to yield two novel ACE-inhibiting peptides, whose amino acid sequences were Pro–Pro–Glu–Ile–Asn (PPEIN)(κ-CN; f156–160) and Pro–Leu–Pro–Leu–Leu (PLPLL) (β-CN; f136–140), respectively. The molecular weight and IC 50 value of the peptides were 550 Da and 566.4 Da, and 0.29 ± 0.01 mg/ml and 0.25 ± 0.01 mg/ml, respectively.