Objective To explore whether bone marrow mesenchymal stem cells could inhibit epithelial-mesenchymal transition(EMT) of rat type Ⅱalveolar epithelial cells through inhibiting Smad3 phosphorylation, and play a role on anti pulmonary fibrosis. Methods Rat type Ⅱ alveolar epithelial cell line RLE-6TN cells were cultivated, induced by 5μg/L transforming growth factor(TGF)-β1 in vitro, and divided into normal control group. TGF-β1 induced group and BMSCs intervention group. Morphological changes of epithelial-mesenchymal transition in RLE-6TN cells were observed under an inverted phase contrast microscope. Immunofluorescence methods were used to determine the location of E-cadherin (E-cad) and α-smooth muscle actin（α-SMA). The expressions of E-cad, α-SMA, p-Smad3 and Snail1 were detected by Western blotting in each group. Results The process of EMT induced by TGF-β1 was accompanied by morphological alteration. RLE-6TN cells had an epithelial cobblestone-like morphology, a round or polygonal shape and exhibited very close cell-cell proximity, while after exposure to TGF-β1, the cells displayed spindle-shaped elongated fibroblast-like cells morphology. Concomitant with a down-regulation of the epithelial markers E-cad, the expression of interstitial cells markers α-SMA up-regulated. Similar morphological changes were noticeably suppressed in the BMSCs intervention group, and the BMSCs intervention group showed a reduction in TGF-β1-induced up-regulation of α-SMA, while a down-regulation of E-cad was observed. Compared with control group, the expression levels of p-Smad3 and Snail1 increased significantly after the stimulation of TGF-β1. After BMSCs intervention,the expressions of p-Smad3 and Snail1 significantly decrease compared to the TGF-β1 group. Conclusion BMSCs may block TGF-β1/Smad3 signal transduction pathway, and then inhibit epithelial mesenchymal transition of type Ⅱ alveolar epithelial cells.