Abstract: Grass carp skins were hydrolyzed using alcalase to obtain crude protein hydrolysate, which was then fractionated by molecular mass into four fractions-> 10, 5~10, 3~5, and < 3 k Da.In vitro antioxidant activities of the crude hydrolysate and its fractions were studied.Results indicated that the 3~5 and < 3 k Da fractions were significantly (p < 0.05) higher in scavenging 1, 1-Diphenyl-2-picrylhydrazyl (DPPH) and 2, 2'-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid) diammonium salt radicals (ABTS) than crude hydrolysate or other fractions, while the > 10 k Da fraction had the highest Fe2 +chelating activity.However, the > 10, 5 ~ 10, and 3 ~ 5 k Da fractions were weaker superoxide radical scavengers compared with reduced L-glutathione. The amino acid sequences of the peptide fractions might play a crucial role in their antioxidant activities.These results indicate that the < 3 k Da fraction has the potential to be used as a natural antioxidant in health food.