We aimed to evaluate the effect of melatonin on oxidative stress and ovarian injury in rats. Twenty-four Sprague-Dawley albino rats were divided into three groups: Group 1 as nondiabetic healthy controls (n = 8), group 2 as nontreated diabetic rats (n = 8) and group 3 as melatonin-treated diabetic rats (n = 8). After overt diabetes was produced by intraperitoneal injection of streptozosin, 20 mg/kg/day of melatonin was given intraperitoneally to group 3 for a week. NF-kB and caspase-3 immunoexpressions, lipid peroxidation, the activities of antioxidative enzymes, total oxidant capacity and total antioxidant capacity were assessed. Immunoexpressions of NF-kB and caspase-3 were significantly lower in group 3 than group 2. There was a significant decrease in superoxide dismutase activity in group 2 than group 1 and a significant increase in group 3 compared with group 2. We observed a nonsignificant decrease in catalase activity between group 1 and group 2 and a nonsignificant increase between group 2 and group 3. There was a nonsignificant increase in the plasma level of total oxidant status in group 2 than group 1, but a significant decrease was observed in group 3 compared to group 2. Total antioxidant status was significantly lower in group 2 compared with group 1 and group 3. In conclusion, melatonin ameliorates the negative effects of oxidative stress on DM-related ovarian injury.