Walnut is one of the medicinal plants used in traditional Iranian medicine as a treatment for diabetes, but little scientific documentation supports its antidiabetic action. This study is designed to evaluate the antidiabetic effect of ethanolic walnut leaf extract. Twenty-four male Wistar rats were divided into four groups: nondiabetic rats, alloxan-induced diabetic rats with no treatment, alloxan-induced diabetic rats treated with ethanolic extracts of Juglans regia (200 mg/kg), and alloxan-induced diabetic rats treated with glibenclamide (0.6 mg/kg). Fasting blood sugar decreased meaningfully in diabetic rats treated with J. regia and diabetic rats treated with glibenclamide. Insulin level increased and glycosylated hemoglobin decreased significantly in diabetic groups receiving either glibenclamide or J. regia compared with the diabetic group with no treatment. The histological study revealed that the size of islets of Langerhans enlarged consequentially as compared with diabetic rats with no treatment. Effects of administering glibenclamide or extract of J. regia on all parameters discussed above showed no difference, and both tended to bring the values to near normal. Our data show the ethanolic extract from leaves of J. regia has a dramatic antidiabetic effect on diabetes-induced rats.