This study investigated the therapeutic effects of Inonotus obliqua extract on blood glucose, insulin, and other biochemical parameters in genetically diabetic mice $(C57BL/KsJ^-m^{+/+}Lepr^{db})$. The mice were divided into four groups-control, Chaga 1 (dose of 0.09 mg/kg of body weight), Chaga 5 (5 times of Chaga 1), and Chaga 10 (10 times of Chaga 1) - according to supplemented dose. Inonotus obliqua extract was orally administered to the animals for 6 weeks. The body and organ (liver and kidney) weights were not different among groups. Fasting blood glucose level was significantly lower in the Chaga 5 group compared with the control (p < 0.05). Hemoglobin A1c content was significantly lower in the Chaga 5 group compared with either the control and Chaga 1 group (p < 0.05). There was no significant difference in serum insulin level among groups. The glucose-6-phosphatase activity in liver was significantly the lowest in Chaga 10 group and was significantly lower in Chaga 5 group as compared with those of control and Chaga 1 groups. Therefore, the results of this study demonstrate that Inonotus obliqua extract alleviates many of the symptoms of diabetes in genetically obese mice and may offer a possibility as a therapeutic supplement for the normalization of blood glucose levels in human with hyperglycemia and have beneficial effects in patients with non-insulin-dependent diabetes mellitus.