Abstract: Polymannuronates and polyguluronates were prepared from alginate,and subsequently treated with hydochloric acid for 1 h,2 h and 6 h to obtain mannuronic acid oligomers( M-1、M_2 and M_3) and guluronic acid oligomers( G_1、G_2 and G_3). Besides,carnosine and mannose were used as control to evaluate the scavenging effect of oligosaccharide on the DPPH,superoxident radical and hydroxyl radical. The results showed that Polymannuronates and Polyguluronates had strong scavenging effects on DPPH and superoxident radical,and the scavenging effect became more efficient with the increasing concentration of oligosaccharide. M_3 had stronger effects than carnosine on scavenging DPPH.G_3 was similar with carnosine referring to scavenging effect.M_3 had a greater effect than M_2 and M_1 on scavenging superoxident radical,while G_3 was slightly lower than carnosine.Polymannuronates and Polygluronates showed less effect on scavenging hydroxyl radical than carnosine and mannite.According to the data,the average degree of polymerization of alginate oligosaccharides had a greater effect on antioxidant ability.