Abstract: In order to study the active fraction of oyster protein hydrolysate, it was preliminary separated and purified by using the ultrafiltration membrane and different antioxidant in vitro and amino acid assays were employed. It was found that the ultrafiltrate (<4000u) acted as the scavenger for hydroxyl radical (IC50 17. 29mg/mL) , 1, 1-diphenyl-2-picrylhydrazyl (DPPH·) (IC50 2. 82mg/mL) and superoxide anion radical (IC50 7. 02mg/mL) . And it also had the ability to inhibit lipid peroxidation in a linoleic acid emulsion system. The ultrafiltrate (<4000u) played a higher reducing power to equal concentration of hydrolysate and other ultrafiltrate fractions. Seven amino acids which may play an important role in antioxidant capacity, including Histidine, Proline, Methionine, Cysteine, Tyrosine, Tryptophan and Phenylalanine, were enriched by ultrafiltration. The main component with high antioxidant capacity was arranged in 550 ～ 3890u in the ultrafiltrate (<4000u) .