A water-soluble carboxymethylated derivative （CM-HNP） of Hunai polysaccharide （HNP）from the sclerotia of Pleurotus tuber-regium （Fr.）Sing. was prepared by the reaction of HNP with monochloroacetic acid. The antioxidative activities of CM-HNP were also studied. The results showed that CM-HNP can protect liver mitochondria from lipid peroxidation induced by Fe2+-Vit C including the increase of TBARS contents, the swelling of mitochondria and the decrease of membrane fluidity with a dosage-effect manner. In addition, CM-HNP can effectively scavenge O-·2 generated by the self-oxidation of pyrogallic acid.