The study of herb-drug interactions is among the newest areas of research affecting the modern practice of medicine. Garlic is an important ingredient of food and many people use garlic bulb for prophylaxis of disease and treatment of various diseases. Daily use of garlic may interact with drugs used in treatment of various diseases. The present study is designed to evaluate the effect of Garlic on metformin in Streptozotocin induced diabetic rats. Metformin was given orally in two different doses of 50mg/kg and 100mg/kg. Allium sativum extract (ASE) was administered at a dose of 500 mg/kg. The Blood glucose was estimated at 0,7,14,21and 28 days. Body weight of the rats of all the groups was recorded before and after the study period of 28 days. All the treatments showed significant (p<0.01) reduction in blood glucose when compared to Diabetic control group. ASE alone or also in combination with metformin improve the body weight of diabetic rats. The study concluded that ASE potentiate the hypoglycemic effect of metformin in diabetic rats i.e. ASE along with metformin showed synergistic effect. This synergistic effect may be helpful in reducing the dose of metformin in treatment of diabetes, which will be helpful in minimizing the adverse effect related to metformin. [J Intercult Ethnopharmacol 2013; 2(2.000): 81-84]