It has been reported that oxidative stress is increased in diabetes mellitus and the increase in oxidative stress gives rise to complications of this disease. In the present study, we aimed to investigate the effects of vitamin A, E, and C treatments combined with insulin on the oxidative and antioxidative systems in the liver, kidney and heart tissues rats in diabetes induced with streptozotocin (STZ). Five experimental groups, each of 10 rats, were constructed as nondiabetic control group, group 1; treated with 3 IU/day s.c insulin, group 2; treated with 3 IU/day s.c insulin + 15 mg/kg/day p.o. vitamin A, group 3; treated with 3 IU/day s.c insulin+ 5.25 mg/kg/day p.o. vitamin E, and group 4; treated with 3 IU/day s.c insulin + 300 mg/kg/day p.o. vitamin C for 30 days. At the end of the study, the blood glucose level in the group 4 was decreased as compared to initial level (p<0.05). The thiobarbituric acid reactive substance (TBARS) levels in the kidney tissue was increased in all groups as compared to control group (p<0.05). As compared to control group, the TBARS level in the liver was increased in groups 2 and 4 (p<0.05), and the heart TBARS level was increased in groups 1 and 3 (p<0.05). The liver lipid hydroperoxide level was decreased significantly in group 4 as compared to groups 1 and 2 (p<0.05). The liver glutathione peroxidase (GSH-Px) level was decreased in group 4 as compared to control group and group 1 (p<0.05), and also liver superoxide dismutase (SOD) level was decreased in group 4 as compared to control group, group 1 and group 2 (p<0.05). The heart catalase and SOD levels were increased in the groups 1 and 2 as compared to control group (p<0.05), and also heart catalase level was decreased in the groups 3 and 4 as compared to group 1 (p<0.05). Results of our study indicate that insulin plus vitamin C treatment may provide more benefits than use of only insulin or insulin plus vitamin A and E treatment in decreasing the blood glucose levels and the liver lipid peroxidation.