Background Diabetic nephropaty (DN) occurs in approximately 40% of patients with diabetes mellitus, and is the most common cause of end-stage renal disease. The mechanisms of DN are not very clear more recently, but the renin-angiotensin system (RAS) plays an important role. It has been known that rennin-angiotensin system blockers have a renal protective effect. The present study was undertaken to evaluate the effects of the candesartan on functional renal tests in streptozotocin (STZ) induced DN in rats. Methods DM was induced by a single intraperitoneal injection (ip) injection of STZ (60 mg/kg). In order to develop DN the animals were left in diabetic condition during 4 weeks. The DM rats were randomly assigned to the two experimental groups (STZ and STZ+CAN). To estimate the symptoms and signs of DN, the STZ group of rats was left without treatment. For assessment of the effects of the AT1 antagonist, candesartan (CAN) (5 mg/kg/day) was administered from week 4 to week 12. Renal function was assessed by determination of serum creatinine, blood urea nitrogen (BUN) test, 24-hour urine volume and urine albumin. Results The administration of STZ have caused diabetes mellitus (DM) with symptoms and signs of DN including poor general condition, body weight loss, as well as abnormalities of serum and urinary renal function tests. In STZ group of rats, these symptoms have been more distinctly 8 and 12 weeks after administration of STZ. Conclusion The candesartan treatment, although not completely but to a great extent ameliorates the functional renal disorder induced by STZ and may be used as a first line drug in preventing DN.