Tubulointerstitial fibrosis (TIF) is a common pathological feature of end-stage kidney disease. Previous studies showed that upregulation of TGF beta 1 notably contributed to the chronic renal injury and irbesartan halted the development of TIF in rats with 5/6 renal mass reduction. This study was to investigate the effects of irbesartan on chronic TIF and the mechanism involved TGF beta 1 in the rodent model of chronic renal failure involving 5/6 nephrectomy. The results showed that irbesartan significantly attenuated the rise in blood pressure and tubulointerstitial injury observed in this model. Masson staining of the renal tissue revealed that there appeared severe renal tubule atrophy and fibrosis in operation group, but the lesion was attenuated mostly in irbesartan-treated group. Immunohistochemistry showed that irbesartan treatment apparently decreased the protein expression of TGF beta 1 which was up-regulated in operation groups. Western blot showed that irbesartan treatment down-regulated the expression of TGF beta 1, phosphorylated smad2 (p-smad2), AT1R and phosphorylated p38 (p-p38) MAPK, but significantly up-regulated the protein expression of smad6 as compared with operation group. These findings suggest that irbesartan attenuates hypertension and reduces the development of TIF in rats with 5/6 renal mass reduction via changes in the expression of these proteins at least including smad6, TGF-beta 1, p-smad2, AT1 and p-p38 MAPK.