In this study, the antioxidant activity of chitooligosaccharides (COSs) less than 10 kDa in molecular weight was characterized in the liver of rats fed a high fat diet (n=5, age 4 weeks). The enzyme activities of superoxide dismutase (SOD), glutathione peroxidase (GPx), glutathione S-transferase (GST), and catalase (CAT) in the liver were measured after feeding rat a high fat diet (high fat group) or a high fat diet supplemented with 0.2 g of COSs per kg of rat body weight for 4 weeks. The activity of SOD, GPx, GST and CAT for the COSs group was higher than that of high fat group. These results indicate that the COSs of less than 10 kDa have potent antioxidant activity in rats fed with high fat diet.