The effects of ultrafiltration on Angiotensin-I converted enzyme(ACE)inhibitory activity,molecular weight distribution and composition of Mytilus edulis protein hydrolysates were investigated.The results showed that the optimum conditions of the first-degree ultrafiltration are:pressure 138 kPa,temperature 20℃ and concentration 15g/L.The permeate fractions of 10 KD membrane showed higher ACE inhibitory activity than original hydrolysates.Meanwhile,ultrafiltration can effectively cut off the polysaccharides and concentrate the ACE inhibitory peptides.However,the effect of ultrafiltration on molecular weight distribution was not as obvious.