The abilities of two types of chitosan oligosaccharides, chitosan oligosaccharide I (1-kDa<MW<3-kDa) and chitosan oligosaccharide II (3-kDa<MW<5-kDa), to prevent 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced oxidative stress were examined in ICR mice. Chitosan oligosaccharide I had no effect on TCDD-induced alterations in lipid peroxidation and glutathione S-transferase activity or liver weight change. However, mice treated with chitosan oligosaccharide II were protected from TCDD-induced lipid peroxidation, inhibition of glutathione peroxidase and glutathione S-transferase activities, and losses in body and liver weights. These results suggest that chitosan oligosaccharide might be a potential agent for combating TCDD-induced pathogenesis.