Chitosan with 80% degree of deacetylation was hydrolyzed by cellulase of Trichoderma viride to prepare chitooligosaccharides (COSs) by the fractionation of the COSs with ultrafiltration membrane. The antioxidant activities of the COSs were clarified in this study by reducing power and free radical scavenging ability assay by UV-VIS absorption spectrum. The results show that the COS 1 (10,000-5,000 Da), COS 2 (5,000-3,000 Da), COS 3 (3,000-1,000 Da) and COS 4 (less than1,000 Da) segments have antioxidant properties.The antioxidant activitives of the COSs increased with the increment of concentration, and they also depended on molecular weight.