Objective To investigate the protective effect of N-acetylcysteine(NAC) on the development of myocardial hypertrophy in diabetic rats induced by streptozotocin. Methods Male SD rats were randomly divided into normal control (NC), control treatment (NCT), diabetic (DM) and diabetes treatment (DMT) groups. NAC was administered at dose of 1.4～1.5g·kg~(-1)·d~(-1) to NCT and DMT groups in the drinking water for 8 weeks. At termination the rats were surgically prepared for hemodynamic measurement, such as systolic blood pressure, heart rate, the rate of left ventricle relaxation (-dp/dt) and the time to maximum relaxation (T). Subsequently, the hearts were removed to assay relative ratio of left ventricle weight versus body weight (LVW/BW) and cardiomyocyte cross sectional areas.Plasma glucose, insulin, 15-F_(2t)-isoprostane(15-F_(2t)-Isop) concentration and superoxide dismutase (SOD) activity were analyzed. Results The LVW/BW ratio and cardiomyocyte areas were significantly attenuated by NAC treatment as compared to DM group (P＜ 0.05). Increases in -dp/dt and SOD activity and the decreases in glucose, 15-F_(2t)-Isop T (P＜ 0.05) were found at the same time. Conclusions NAC can effectively prevent the development of myocardial hypertrophy and improve diastolic function in diabetic rats, which may be related to its antioxidant capacity.