Separation effect of protein hydrolysates by ultrafiltration membrane system with 3,000, 6,000, 10,000, 30,000 Daltons molecular-weight-cut-off (MWCO) membranes, and the inhibitory ability (I%) of angiotensin converting enzyme (ACE) of filtrate fraction by Sephadex G-25 and Sephadex G-15 filtration chromatography were investigated, respectively. The results showed that different composition could be separated from hydrolysates with different MWCO membranes and different size peptide had great different ACE inhibitory ability. The smaller MWCO of ultrafiltration membranes, the higher ACE inhibitory ability of the fraction. The filtrate from the 3,000 Daltons MWCO membrane had a much higher ACE inhibitory ability, I％=96.17％ and IC50=0.078 mg·mL-1, than that from other MWCO membrane. The average molecular of two group peptides separated by Sephadex G-25 and Sephadex G-15 filtration chromatography followed by separation from the hydrolysates with 3,000 Daltons MWCO membrane, which possessed the highest ACE inhibitory ability (IC50 0.026 mg·mL-1 and 0.012 mg·mL-1), were 1,300 and 900 Daltons, respectively.