Objective To observe the expression of MicroRNA-21(miRNA-21)in kidney tissue of unilateral ureteral obstruc-tion (UUO)rats and explore mechanisms of miRNA-21 involved in TGF-β1/Smad3 signaling pathway in renal interstitial fibrosis (RIF).Methods Totally 20 adult male SD rats were divided into UUO group and Sham operation group.The left ureter of rats in UUO group were ligated,and the rats in sham group were dissociated left ureter only,but not to be obstructed.The left kidney tis-sue was collected at the 3rd、7th days after surgery.Real-time quantitative PCR analysed the expression of miRNA-21.Masson stai-ning,HE staining and immunohislochemistry staining were used to evaluate the degree of RIF.The expression of transforming growth factor-β1(TGF-β1)、Smad3,α-smooth muscle actin (α-SMA)、and collagenⅠ(Col-Ⅰ)were examined by immunohislochemis-try staining.Results The expression of miRNA-21 in kidney tissue was higher at 3rd、7th days after surgery in UUO groups than that in Sham group(P <0.01).The expression in UUO group at 7th day was higher than 3rd day after surgery(P <0.01).The fi-brosis score,positive TGF-β1、Smad3、Col-Ⅰ、α-SMA area in UUO group were significantly higher than those of Sham group at 3rd、7th days (P <0.01),Which at 7th day were higher than 3rd day in UUO group(P <0.01).The expressions of miRNA-21 in kidney tissue was positive correlations with the fibrosis score(r=0.888,P <0.01).The expressions of miRNA-21 in kidney tissue was positive correlations with TGF-β1、Smad3、Col-Ⅰ、α-SMA(r=0.799,0.849,0.882,0.896,P <0.01).Conclusion The expres-sion of miRNA-21 was upregulated in UUO kidney tissue after model establishment.The RIF degree might be positive correlations with miRNA-21 expression,and TGF-β1/Smad3 signaling pathway positively regulate miRNA-21 to mediate rats RIF.