Background: Oxidative stress and impaired antioxidant capacity in diabetes are associated with diabetic nephropathy. Metformin, as an adjunct to insulin could decrease oxidative stress and may therefore improve renal function in type 1 diabetes (T1D). Objectives: To investigate the effects of metformin as adds-on therapy to insulin on renal dysfunction in T1D. Materials and Methods: Male Sprague-Dawley rats (230-250 g) were divided into 5 groups (n =7). Rats in groups A and B were orally treated with 3.0 mL/kg body weight (BW) of distilled water, while those in groups C and D were treated with insulin (4.0 U/kg BW bid) or oral metformin (250 mg/kg BW), respectively. Group E rats were similarly treated with both metformin and insulin. Groups B-E were rendered diabetic by intraperitoneal injections of 65 mg/kg BW of streptozotocin. Fasting blood glucose concentrations and glucose tolerance tests were done. The animals were sacrificed by halothane overdose after 56 days, blood taken by cardiac puncture and kidneys excised and stored at -80°C for further analysis. Results: Untreated diabetic rats exhibited significant weight loss, increased polydipsia and polyuria, impaired glucose tolerance, electrolyte retention, reduced creatinine clearance and urea excretion and increased oxidative stress compared to controls, respectively. However, these were reversed by treatment with metformin and insulin. Conclusions: Metformin does not improve glycemic control in TID but exerts renoprotective effects by reducing oxidative stress in the presence of insulin. Metformin should therefore be considered for adjunct therapy with insulin in TID.