This study was performed to investigate the effect of Puerariae radix-ethanol extracts rich in isoflavone on the antio-xidative system of rats. For this purpose, first, Puerariae radix was extracted with ethanol, and its total isoflavone and puerarin contents were analysed. Second, female Sprague Dawley rats were fed for 6 weeks with four diets which were based on AIN96G diet and supplemented with Puerariae radix-ethanol extracts to contain isoflavone. The isoflavone contents of four experimental diets were 0 mg, 500 mg, 1,000 mg, 2,000 mg per kg diet, respectively (control, P0.05%,P0.1%, P0.2%). Liver and erythrocyte activities of antioxidative enzyme such as superoxide dismutase (SOD), catalase,glutathione peroxidase (GSHpx) were measured. Also, plasma and liver malondialdehyde (MDA) concentrations, liver glutathione (GSH) and oxidized glutathione (GSSG) concentrations were measured. The total isoflavone content of Puerariae radix-ethanol extract was 3067.6 mg per 100 g extract and the content of puerarin was 2557.4 mg per 100 g extract. The erythrocyte activities of GSH-Px and catalase were higher in group P0.1% and P0.2%. But SOD activity of erythocyte did not show any difference by the Puerariae radix-ethanol extract supplementation in diet. The activity of SOD in liver increased significantly by the supplementation of extract, showing highest level in P0.1% group. The liver GSH concentration increased significantly in group of P0.05%, P0.1%, and P0.2% compared with control group (p <0.05). The GSSG concentration in liver showed no difference by the supplementation of Puerariae radix extract from the control group, except P0.2% group. The plasma MDA concentration did not show any significant differences by the extract supplementation. But the liver MDA concentration decreased by the extract supplementation, showing the lowest level in P0.1 % diet group. These results suggest that the supplementation of Puerariae radix-ethanol extract can inhibit lipid peroxidation in liver and enhance the antioxidative defense competence of rats