Objective To explore the effects of Sinomenine (Sin) on phenotype transdifferentiation in renal tubular epithelial cells. Methods Mice primary tubular epithelial cells and cell line (MCT) were cultured in vitro. Model group cells were stimulated by IL-1?(10 ng/ml). The cells of Sin group were co-incubated with Sin(10, 100 ,500?mol/L) simultaneously. The cells without IL-1?and Sin treatment were regarded as control group. The protein expression of?-smooth muscle actin (SMA) and Vimentin was assessed by immunohistochemistry and Western blotting. The mRNA expression of?-SMA and Vimentin was examined by RT-PCR. Secretion of matrix metalloproteinases was determined by zymography. Results The mRNA expression and protein level of?-SMA and Vimentin, and the enzyme activity of MMP-2 and MMP-9 in tubular epithelial cells stimulated with IL-1?were significantly higher than those in the controls. Sinomenine significantly reduced the expression levels of protein and mRNA of?-SMA and Vimentin. At the same time, the MMP-2 and MMP-9 activities were reduced markedly. Conclusion Sinomenine can decrease the transdifferentiation in renal tubular epithelial cells stimulated by inflammatory factor IL-1?and prevent renal interstitial fibrosis.