Objective：To investigate the effect of the tetrahydrobiopterin，which is known to be an important cofactor of nitric oxide synthase（NOS） and an antioxidant，on development of diabetic cardiomyopathy in db/db mice and the underlying mechanism. Methods：Db/db mice were treated with phosphate buffer solution（PBS），low dosage tetrahydrobiopterin（BH4） [10 mg/（kg·d）] or high dosage BH4[50 mg/（kg·d）] for 8 weeks by gavages. C57 mice were used as normal control. Cardiac function was measured by inserting a Millar catheter into the left ventricle. Then，myocardial H2O2 level，malondialdehyde（MDA） level and superoxide dismutase（SOD） activity were detected in every group. Results：The cardiac function in diabetic group was significantly suppressed compared with that in normal control group. Meanwhile，compared with those in normal control group，H2O2 and MDA in diabetic group were both increased. However，no difference was observed in SOD activity between normal control group and diabetic group. After treatment of BH4，the cardiac function was improved in both low dosage and high dosage BH4 groups than in diabetic group. In addition，treatment with BH4 attenuated the H2O2 and MDA levels and increased the SOD activity. Conclusion：The data suggest that BH4 can significantly improve cardiac function of db/db diabetic mice，attenuate the oxidative stress and improve antioxidant capacity.