Objective: To explore the possible mechanism of HuaYu JieDu Tang in inhibiting renal interstitial fibrosis in rat. Methods: Rat renal interstitial fibrosis model was produced by unilateral ureteral obstruction(UUO). Collagen Ⅰ and collagen Ⅲ of rat renal interstitial tissue were detected by immunohistochemical methods. Transforming growth factor-β 1(TGF-β 1)mRNA and Matrix metalloproteinase 1(MMP-1)mRNA of rat renal interstitial tissue were detected by situ hybridization methods. Urinaryα1-MG was measured and correlated with renal histology. Results: The surface area rate of collagenⅠ, collagen Ⅲ and TGF-β1 mRNA, MMP-1 mRNA of model and all treated rats were obviously higher than that of normal rats (P<0.05). The surface area rate of collagenⅠ and collagen Ⅲ and TGF-β1 mRNA of all treated rats were obviously lower than those of model rats(P<0.05). MMP-1 mRNA expression was higher in treated group than that of model group. The renal histology in all treated rats were better than that of model rats(P<0.05). Conclusion: HuaYu JieDu Tang can inhibit the ccumulation of ECM in renal interstitium and prevent renal interstitial fibrosis by down-regulating TGF-β1 mRNA and up-regulating MMP 1mRNA expression. It also inhibits renal interstitial fibrosis by decreasing renal tubule lesion. -β1 mRNA and up-regulating MMP 1mRNA expression. It also inhibits renal interstitial fibrosis by decreasing renal tubule lesion.