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Aim: Myofibroblasts play important roles in the pathogenesis of lung fibrosis.Transforming growth factor (TGF)-β1 has been widely recognized as a key fibrogenic cytokine. The major signaling pathway of TGF-β1 is through cytoplas-mic Smad proteins. Our study investigated the role of individual TGF-β1/Smad signal proteins in mediating α-smooth muscle actin (α-SMA) gene expression,which is a well-known key marker of myofibroblast differentiation. Methods: We transiently cotransfected α-SMA promoter-luciferase fusion plasmid (p895-Luc)and Smad expression plasmids and measured Luc activity in TGF-β1-treated hu-man fetal lung fibroblasts. We induced Smad3 knockout mice lung fibrosis by bleomycin, α-SMA protein expression was assessed by Weste blotting. Col-lagen protein was analyzed by measuring hydroxyprolin. Myofibroblast morphol-ogy was assessed by immunohistochemistry. Results: We found that the overexpression of Smad3, not Smad2 markedly increased TGF-β1-induced α-SMA promoter activity and α-SMA protein expression in vitro, whereas the over-expression of dominant negative mutant Smad3 and Smad7 repressed TGF-β1 induced α-SMA gene expression. Compared to wild-type mice, Smad3 knockout mice showed attenuated lung fibrosis after bleomycin treatment, manifested by lower collagen production and myofibroblast differentiation. Conclusion: Our study suggested TGF-β1/Smad3 is a major pathway which regulated the myofibroblast differentiation. This result indicates a potential significance for future attempts of attenuating the progression of human lung fibrosis by the inhibition of the Smad3 cascade.