Epidemiological studies have revealed positive correlation between particulate matter with an aerodynamic diameter of < 2.5 mu m (PM2.5) and pulmonary fibrosis (PF). As etiology and pathogenesis of PF have not been fully elucidated, this study was to investigate the potential mechanism by which PM2.5 exposure adversely induced PF in vivo and in vitro. In the present study, 6-week-old C57/BL6J mice were intranasally administrated with PM2.5 (100 mu g/day) for 4 weeks. Micro-CT and hematoxylin-eosin (HE) staining analysis showed that lung inflammation and incipient fibrosis symptoms were induced after PM2.5 exposure. The expression of Transforming growth factor-beta 1 (TGF-beta 1), alpha-Smooth muscle actin (alpha-SMA), and Collagen type I (COL1) in mice lung was increased. Upregulation of TGF-beta 1 in mice serum was also detected by ELISA after exposure to PM2.5. Moreover, chronic PM2.5 exposure on human bronchial epithelial cell line BEAS-2B cells led to activation of TGF-beta 1/SMAD3 pathway, TGF-beta 1 excretion and epithelial-mesenchymal transition (EMT), while PM2.5 also triggered the activation of TGF-beta 1/SMAD3 pathway, TGF-beta 1 excretion as well as differentiation of human pulmonary fibroblast cell line HFL-1 cells, and TGF-beta 1 production in mouse macrophage cell line RAW264.7 cells. Furthermore, cell culture medium of PM2.5-treated BEAS-2B and RAW264.7 cells could both activate TGF-beta 1/SMAD3 signaling, alpha-SMA and COL1 upregulation in HFL-1 cells. Therefore, we concluded that PM2.5 could induce PF by targeting pulmonary epithelium, macrophages and fibroblasts, suggesting that PM2.5 was a potent initiator of PF.