flow. Losartan is angiotensin II receptor I (AT1) antagonist and is used for treatment of congestive heart failure and hypertension. It is widely recognized that Losartan has organ protective nature and most effective for organ damage progressing. The aim of this study was to evaluate the effect of Losartan on apoptosis in renal tissue after unilateral ureteral obstruction in rat. Materials and Methods: In this experimental study, adult male Sprague-Dawley rats were randomly divided into five groups (ten rats in each group) as follows: (1) control (2) unilateral ureteral obstruction (UUO) (3) UUO/Losartan (UUO/LOS) (4) Sham-operated (5) Sham/LOS. Control animals received drug solvent. Unilateral ureteral obstruction was performed in groups 2 and 3 and sham operations were performed in groups 4 and 5. In group 2, animals received drug solvent and in group 3 animals received Losartan (60 mg/kg). All drugs administered orally for 15 days (started before operation). Apoptosis in renal tissue were studied in left renal in different groups with tunnel method at day 14. Results: Tunnel staining determined that experimental unilateral ureteral obstruction caused induction of apoptosis (15.52±1.33) in tubular cells of renal tissue but, in Losartan treated animals number of apoptotic cells (5.24±0.93) significantly (p<0.05) decreased. There was no significant difference between control (0.91±0.26), sham (1.17±0.29) and sham/LOS (2.16±0.47) groups. Conclusion: Our results showed that experimental unilateral ureteral obstruction induces apoptosis in renal tissue but, Losartan administration decreased the number of apoptotic cells in renal tissue.