Objective    To investigate the inhibition of interstitial fibrosis by NCTD is related to the dephosphorylation of Smad3 linker region mediated by the inhibition of PP2Ac.  Methods   HK -2 cells were cultured and devided into 5 groups: (1) normal control group; (2) TGF-β1 group (5 μg/L); (3) TGF-β1+NCTD group (2.5 mg/L); (4) TGF-β1+PP2Ac shRNA group; (5)TGF-β1+PP2Ac shRNA+ NCTD group. Real-time PCR and Western blot were used to detect the expression of PP2Ac, FN, Col-I, α-SMA and E-cadherin. Additionally, the HK-2 cells were assigned to three groups:(1) normal control group; (2) TGF-β1 group; (3) TGF-β1+NCTD group. Immunofluorescence were used to analysis the distribution of pSmad3-L(Ser²⁰⁴) and pSmad3-L(Ser²⁰⁸). Western blot analysis were used to detect the protein expression of pSmad3-L(Ser²⁰⁴) and pSmad3-L(Ser²⁰⁸).  Results   (1) TGF-β1 stimulated the expression of PP2Ac in HK-2 cells, increased the expression of FN, Col-I and α-SMA, and decreased the expression of E - cadherin. Both NCTD and PP2Ac shRNA could inhibit PP2Ac expression accompanied with the downregulation of FN, Col - I and α - SMA, and upregulation of E - cadherin. However, compared with PP2Ac shRNA transfected group, cells transfecting with PP2Ac shRNA and incubated with NCTD showed no obvious differences on the relief of the above indicators induced by TGF-β1 in HK2 cells. (2)The expression of pSmad3-L(Ser²⁰⁴) and pSmad3-L(Ser²⁰⁸) in the nucleus of HK2 cells stimulated by TGF-β1 was significantly elevated. The expression of pSmad3-L(Ser²⁰⁴) and pSmad3 - L(Ser²⁰⁸) in the nucleus was further upregulated when treated with NCTD.  Conclusions   NCTD has anti - fibrosis effect and it may be due to the inhibition the dephosphorylation of Smad3 linker region mediated by the inhibition of PP2Ac.