Abstract: To investigate the antioxidant effects of the Natto polysaccharide on the liver and kidney of immuno-suppressive mice induced by cyclophosphamide and find the optimum concentration. A total of 40 Kunming mice weighing （20±2） g were randomly divided into four groups, namely, normal control group, cyclophosphamide treatment model group, high-dose Natto treatment group （500 mg/kg） and low-dose Natto treatment group （200 mg/kg）. All mice in the groups except normal control group were administrated with cyclophosphamide at the dosage of 80 mg/kg, and then the two Natto treatment groups were given Natto polysaccharide at the dosage of 500 and 200 mg/kg by garage, respectively for two weeks. The results showed that the Malondialdehyde （MDA） contents of liver and kidney in Natto treatment groups were both significantly decreased （P 〈 0.05） compared with that of cyclophosphamide treatment model group, but the superoxide dismutase （SOD） contents of liver and kidney in Natto treatment groups were greatly increased compared with that of cyclophosphamide treatment model group. Natto polysaccharide has antioxidation function and the optimum dose is 200 mg/kg in mice.