This study evaluated the protective effect of Puerariae radix against the oxidative stress induced by hydrogen peroxide (H2O2) and streptozotocin in vitro and in vivo, respectively. The ethanol extract scavenged intracellular reactive oxygen species (ROS), and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical, and prevented lipid peroxidation. This radical scavenging activity of the ethanol extract protected the cell viability of Chinese hamster lung fibroblast (V79-4) cells exposed to H2O2. Furthermore, this extract reduced the formation of apoptotic cells induced by H2O2, which was demonstrated by the decreased number of sub G(1) hypo-diploid cells and apoptotic cell body formation. The extract increased the activities of the cellular antioxidant enzymes, superoxide dismutase (SOD) and catalase (CAT). Administration of the extract to the streptozotocin induced diabetic rats decreased the blood glucose levels. The diabetic rats showed low activities of superoxide dismutase and catalase in the liver, and the ethanol extract increased the CAT activity. The increased level of lipid peroxidation in the diabetic rats reverted to near normal levels after being treated with the extract. This study showed that Puerariae radix was effective in the amelioration of diabetes, which may be a consequence of its antioxidant potential.