To investigate the effects of mycophenolate mofetil MMF) on the process of renal inter-stitial fibrosis,unilateral ureteral obstruction (UUO) model was established in rats. Twenty Spra-gue-Dawley rats underwent UUO and received vehicle (n= 10) or MMF (20 mg. kg-1. d-1 , by dai-ly gastric gavage, n= 10) during a period of 5 days following surgery, and the additional 10 ratswere served as sham-operated group. The rats were killed 5 days after surgery. Immunohistochem-istry was performed on renal tissue for proliferating cell nuclear antigen (PCNA), a-smooth muscleactin (α-SMA) and type Ⅰ and Ⅲ collagen (col Ⅰ , colⅢ). Histological studies were also done byMASSON staining. Five days after surgery, proliferating cells in tubules, interstitium as well as in-terstitial myofibroblast (MyoF) infiltration and interstitial col Ⅰ , colⅢ deposition were all signifi-cantly reduced by MMF treatment. MMF also alleviated the histological changes of UUO rats.These results suggested that the reduction of interstitial MyoF infiltration may be an important e-vent by which MMF prevents renal injury caused by UUO and MMF could be used to limit the pro-gression of renal fibrosis.