Introduction: Diabetes, as a chronic disease, is the third leading cause of death in developing countries. Hyperglycemia and oxidative stress have been recognized as the main factors involved in pathogenesis of diabetes. On the other hand, the antioxidant system is the first defense mechanism of body against oxidative stress. Falcaria Vulgaris possesses hypoglycemic and antioxidant effects. This study surveyed the effects of different doses of Falcaria vulgaris extract (50,100,150 mg/kg) on histological changes of Langerhans islets andserum insulin, nitric oxide and glucose levels. Materials and Methods: A total of 64 male Wistar rats were divided into 8 groups (control, diabetic with STZ, treatment with Falcaria Vulgaris (50,100,150 mg/kg) and diabetic treated with Falcaria Vulgaris) (50,100,150 mg/kg). Data were analyzed by one-way ANOVA, and p value<0.05 was considered significant. Results: Falcaria Vulgaris extract (100 and 150 mg/kg) significantly decreased serum glucose level (p<0.01) and improved the diameter of islets (p<0.05) in diabetic rats treated with Falcaria Vulgaris extract, compared with the diabetic group. Moreover, at dose of 150 mg/kg, the extract improvedserum insulin (p<0.01), decreased nitric oxide (p<0.01) and increased the weight (p<0.01) and number of islets of diabetic rats (p<0.05). Histopathological studies also confirmed these changes. Conclusion: F. vulgaris can improve insulin secretion and serum glucose levels in an animal model of STZ induced diabetes, possibly by reducing nitric oxide production and preventing pancreatic tissue oxidative damage.