Alginate-derived oligosaccharides were obtained by enzymatic hydrolysis,and the antioxidant activities of oligosaccharides with different molecular weight range,ADO-A,ADO-B and ADO-C,were studied. The results showed that three kinds of oligosaccharides showed certain antioxidant activities in a concentration dependent manner. Alginate oligosaccharides C showed the highest DPPH radical-scavenging activity,and the IC 50 of ADO-C was 0. 53mg / mL. Under the same concentration,ADO-C showed higher hydroxyl free radical-scavenging activity than V C( P <0. 05). ADO-C also showed significant higher reducing power than ADO-A and ADO-B( P <0. 05). These results showed that the alginate oligosaccharides obtained by hydrolyzation of sodium alginateby alginatelyase had antioxidant activities,wherein ADO-C( M W < 8 000) showed the strongest activity,which indicated that the oligosacchairdes with smaller molecular weight would have better antioxidant activities.