This study investigated the antioxidative action of Corni Fructus aqueous extract on kidneys of diabetic mice. The electron donating abilities of Corni Fructus aqueous extract and its antioxidant activities (XO, SOD, CAT, GST, eNOS) in kidneys of C57BL/6 or db/db mice were evaluated. For in vivo study, seven week-old male mice were divided into normal control group (NC, C57BL/6 mice), diabetic control group (DC, db/db mice) and Corni Fructus (500 mg/kg/day for 8 weeks) treated diabetic group (DCF, db/db mice). The electron donating abilities of Corni Fructus aqueous extract exhibited 7%, 24.4%, and 42.7% at concentrations of 100, 500, and $1000;{mu}g/ml$, respectively. The activity of XO in the DCF group was significantly lower than the DC group by 35% (p < 0.05). The SOD activity was significantly higher in the DCF group than the DC group by 26% (p < 0.05). The activities of CAT and GST were lowered in the DCF group than the DC group by 26% (p < 0.05) and 7.6%, respectively. The mRNA expression of eNOS in kidneys was lower in the DCF group than the DC group by 24%. These results indicate that Corni Fructus reduced oxidation stress as evidenced by the restoration of the enzymatic antioxidative defense system in renal tissues of db/db mice. It is suggested that these antioxidative actions of Corni Fructus on renal tissues in db/db mice could contribute to its renoprotective effects on diabetic nephropathy.