Abstract: Objective To investigate the role of ADAM17 in epithelial - mesenchymal transition (EMT) of A549 cells and the mechanism underlying pulmonary fibrosis. Methods A549 cells were equally divided into four groups: blank control group, TGF-β1-mediated group, PMA (the activator of ADAM 17) group and TNF484 (the inhibitor of ADAM17) group. A549 cells were cultured in vitro, cellular morphology changes after 36 h were observed by phase contrast microscope. The mRNA and protein expressions of ADAM17 and the markers of epithelial cell and mesenchymal cell were determined by Real-time PCR and Western blotting. Results A549 cells lined up tightly in the blank control group, however, after mediated by TGF-β1, the cells became spindle and loosen. Real-time PCR and Western blotting results showed that the expression of ADAM 17 were higher in PMA group and lower in TNF484 group than that in blank control group, which had significant difference. The high expression of E-cadherin in blank control group and TNF484 group and the high expression of Vimentin in TGF-β1-mediated group and PMA group were also of significant difference. Conclusion High expression of ADAM 17 contributes to the EMT of alveolar epithelial cells, which suggests that ADAM 17 can be one of fundamental mechanisms of pulmonary fibrosis.