To study the effects of Rehmanniae Radix and Rehmanniae Radix Praeparata on renal interstitial fibrosis (RIF) of obstructed kidneys in a unilateral ureteral obstruction (UUO) rat model and the mechanism.20 rats were randomly divided into four groups : sham operated group, UUO group, Rehmanniae Radix decoction group and Rehmanniae Radix Praeparata decoction group. The sham operated group and the UUO group were orally administered normal saline, the drug groups were orally administered 1.6 g/(kg x d) of Rehmanniae Radix decoction and Rehmanniae Radix Praeparata decoction for 14 days, respectively. Then the serum of rats were collected for measurements of Scr and BUN contents. Histological changes of kidneys were observed by HE and Masson staining. The expression of transforming growth factor-β1, (TGF-β1), α-smooth muscle actin (α-SMA) and type I collagen (Collagen-I) were detected by Western blot method.Compared with model group, the contents of Scr and BUN in rats serum of drug groups were significantly decreased (P < 0.05), the sclerosis of glomerulus was ameliorated, the degree of saccular dilatation and the damage of tubules were alleviated, collagen distribution and interstitial fibrosis was obviously improved,while the expressions of TGF-β1, α-SMA and Collagen-I were decreased. Compared with Rehmanniae Radix Praeparata decoction group,the contents of Scr and BUN in rats serum of Rehmanniae Radix decoction group were lower, the decrease of collagen distribution and interstitial fibrosis were more obviously, while the expression of α-SMA was significantly decreased (P < 0.05).The effect of Rehmanniae Radix is more powerful than that of Rehmanniae Radix Praeparata on attenuating interstitial fibrosis,which may relative to downregulating the expressions of TGF-β1, α-SMA and Collagen-I.