Objective: To investigate the effect of diabetes mellitus on bone metabolism, bone mineral density (BMD) and serum testosterone were studied in STZ-induced diabetic male rats. Materials and methods: Male Wistar rats were randomly divided into control group (n = 6), diabetic group (n = 5) and insulin-treated diabetic group (n = 5). Diabetes was intravenously induced by injection of streptozotocin (STZ, 50 mg/kg body weight), confirmed by 12 h fasting blood glucose concentration (12 mmol/L). The treatment group was subcutaneously injected with protamine zinc insulin (1.8-2.2U/animal) at the same time every day. The experiment lasted 32 days. Femoral BMD was determined by dual-energy X-ray absorptiometry (DEXA). Serum testosterone was detected by Enzyme-linked Immunosorbent Assay (ELISA) kit. Total alkaline phosphatase (ALP), calcium (Ca) and phosphorus (P) in serum were measured by bio-machine. Results: Femoral BMD in diabetic group was significantly lower than that in control (P < 0.01) but reserved by insulin treatment. Testosterone in serum was also reduced in diabetic rats (P < 0.01) and normalized by insulin treatment. Serum ALP concentration was markedly higher in diabetics (P < 0.01) compared to the controls and prevented by insulin treatment. However, no significant differences were observed in serum Ca and P among three groups. Conclusion: BMD is seriously affected by diabetes. Insulin deficiency and testosterone decrease are important causes of diabetic osteoporosis. And insulin treatment preserved bone loss and testosterone depression.