These studies were performed to investigate the effect of antioxidants, melatonin (MLT) and ascorbic acid (AA), on the diabetic complications. In the experiment, twenty-four Sprague Dawley rats were divided into four groups; Group 1: no treatment (control) (n=6), Group 2: Streptozotocin (STZ; 65 mg/kg b.w. ip) treatment (n=6), Group 3: MLT (10 mg/kg b.w. ip) treatment (n=6), Group 4: ascorbic acid (AA: 40 mg/kg b.w.ip) treatment (n=6). Rats were sacrificed and collected designated samples (blood, kidney, pancreas) at the 21 days after antioxidant treatment (the 8th week after STZ injection). There was no significant change of body weight during the experiment period in the all groups. The concentration of blood glucose was decreased below 200 mg/dl in MLT. The concentration of HbA$_1$c level was decreased significantly in MLT (7.3$pm$0.66%) compared with STZ (15.1$pm$1.58%) (p<0.01). No significant change of SOD activity in the RBC and kidney was found in the all groups. In the pancreas, MLT (1.42$pm$0.17 U/ml) and AA(1.65$pm$0.16 U/ml) decreased SOD activity compared with STZ (4.10$pm$0.27 U/ml). However, there was no significant difference among MLT and AA. The level of malondialdehyde(MDA) as an indicator of lipid peroxydation was decreased in MLT compared with STZ in the plasma, kidney and pancreas(p<0.01). These findings suggest that melatonin could prevent the complications of diabetes mellitus.