The purpose of this study was to investigate the effect of soybean on blood glucose and lipid concentrations, and antioxidant enzyme activity in type 2 diabetes mellitus (DM) patients. We divided patients into two groups and fed them, respectively, a basal diet (control group) and a basal diet with 69 g/d of soybean (soybean group) for 4 weeks. Pills with roasted soybean powder were provided to the soybean supplementation group three times a day. Macronutrients intake except dietary fiber was similar between the two groups. No significant differences were observed in dietary intakes or body weight before and after the supplementation. Energy composition ratio of C:F:P was 65:19:16 in the control group, 64:20:16 in the soybean group. The blood parameters of subjects before supplementation, such as fasting blood glucose, postprandial glucose level, total cholesterol, triglyceride, LDL-cholesterol and HbA₁C were not different between the two groups. After supplementation, fasting blood glucose (p<0.001), postprandial glucose level (p<0.001) and serum triglyceride level (p<0.05) were significantly reduced in the soybean group in comparison with the control group. The total cholesterol level was not significantly different between the control and the supplemented group after 4 weeks of treatment. TBARS levels of the soybean group were not significantly different from those of the control group. The activities of catalase (p<0.01) and glutathione peroxidase (p<0.05) were significantly higher in the soybean group compared to the control group. The results of this study suggest that soybean supplementation would be helpful to control blood glucose and serum lipid in diabetic patients. Also, soybean showed an antioxidant activity that may contribute to enhance the effect of antioxidant defense. This activity contributes to protection against oxidative damage in type 2 DM patients. Soybean may have potential use in the disease management of patients with DM.