Objective: Free radical production is implicated in the pathogenesis of diabetes mellitus, in which several pathways and different mechanisms are shown to involve in the pathophysiology of the complications. Through the administration of nicotinamide(NA) and streptozotocin(STZ), a model of human type II diabetes can be induced in rats. The aim of this study was to determine the effects of Stevia rebaudiana (SrB) and L-NNA (N-nitro-L-arginine) in the formation of free radicals in STZ-NA induced type II diabetic rats. Material and Methods: In this study, rats were treated with SrB and L-NNA 5-8 weeks after the induction of diabetes. The levels of glutathione peroxidase (GPx), superoxide dismutase (SOD), catalase (CAT) and malondialdehyde (MDA) were determined in the liver homogenate and erythrocyte hemolysate. The nitric oxide synthase (NOS) levels were also measured in the liver homogenate and serum. To study the histological changes in diabetes, the liver tissue samples stained with hematoxylein-eosin and were investigated under light microscope. Results: Although fasting and postprandial blood sugar levels were high in diabetic groups, blood sugar levels were significantly reduced in diabetic treated groups. While the erythrocyte MDA levels of stevia-treated diabetic group decreased, lipid peroxidation increased with L-NNA treatment in both control and diabetic groups. There was no significant difference for the tissue CAT, NOS, and erythrocyte SOD and CAT activities between. Liver samples in control group had a normal structure. Normal histological arrangement was noticed in the livers of control group. In hepatocytes of the diabetic control group, necrotic cells with pycnotic nucleus and eosinophylic cytoplasm as well as sinusoidal dilatation were seen. The hepatocyte structure was more protected in diabetic LNNA group than in diabetic group. The maximum protection was seen in hepatocytes of diabetic stevia L-NNA group. Conclusion: SrB and L-NNA reduced blood glucose levels in diabetic rats and had some beneficial effects on oxidative and histological changes. However, a NOS inhibitor, L-NNA was less effective than SrB treatment in type II diabetes.