Etiological studies of diabetes and its complications showed that oxidative stress might play a major role, Therefore, many efforts have been tried to regulate free oxygen radicals for treating diabetes and its complications. Mori Fructus extract has been known to be effective for the antidiabetic, antihyperlipidemic and antiobesitic prescription, and composed of four crude herbs, The aim of this study was to investigate the effect of Mori Fructus extract in male ob/ob mouse with severe obesity, hyperinsulinemia, hyperglycemia, hyperlipidemia. Mice were grouped and treated for 5 weeks as follows. Both the lean (C57BL/6J black mice) and diabetic (ob/ob mice) control groups received standard chow. The experimental groups were fed with a diet of chow supplemented with 7.5, 15 and 30 mg Mori Fructus extract per 1 kg of body weight for 14 days. The effects of Mori Fructus extract on the ob/ob mice were observed by measuring the serum levels of glucose, insulin, lipid components, and the kidney levels of reactive oxygen species (ROS), MDA+HAE, GSH and also the enzyme activities involved in polyol pathway. Western blotting was performed using anti-AGE, anti-RAGE respectively. Mori Fructus extract lowered the levels of serum glucose and insulin in a dose dependent manner. Total cholesterol, triglyceride and free fatty acid levels were decreased, while the HDL-cholesterol level was increased, in Mori Fructus extract treated groups. Renal aldose reductase and sorbitol dehydrogenase activities were increased in the ob/ob mice, whereas those were inhibited in the Mori Fructus extract-administered groups. Mori Fructus extract inhibited the generation of ROS in the kidney. MDA+HAE level was increased and the GSH level was decreased in the ob/ob mice, whereas those were improved in the Mori Fructus extract-administered groups. Mori Fructus extract inhibited the expression of AGE, RAGE in the ob/ob mice. The results suggested that Mori Fructus exerted the antidiabetic and antihyperlipidemic activities by regulating the activities of polyol pathway enzymes, scavenging the ROS, decreasing the MDA+HAE level, increasing the GSH level and inhibiting the expression of AGE, RAGE in the ob/ob mice.