Epithelial-mesenchymal transition (EMT), the transdifferentiation of epithelial cells into mesenchymal cells, has been implicated in the metastasis and provides novel strategies for cancer therapy. Osthole (OST), a dominant active constituent of Chinese herb Cnidium monnieri, has been reported to inhibit cancer metastasis while the mechanisms remains unclear. Here, we studied the inhibitory effect and mechanisms of OST on TGF-b1-induced EMT in A549 cells. Cells were treated with TGF-b1 in the absence and presence of OST. The morphological alterations were observed with a microscopy. The protein and mRNA expressions were determined by Western blotting and realtime PCR. The protein localization was detected with immunofluorescence. The adhesion, migration, and invasion were determined by Matrigel, wound-healing, and Transwell assays. TGF-b1 treatment induced spindle-shaped alterations of cells, upregulation of N-cadherin, Vimentin, NF-kB p65, and downregulation of E-cadherin. Dysregulated membrane expression and mRNA expression of E-cadherin and N-cadherin were observed after TGF-b1 treatment. TGF-b1 increased abilities of migration and invasion and triggered the nuclear translocation of NF-kB p65. These alterations were dramatically inhibited by OST. Furthermore, PDTC, a NF-kB inhibitor, showed similar effects. In addition, TGF-b1-induced expression of Snail was significantly inhibited by OST and silenced Snail partially reversed TGF-b1-induced EMT biomarkers without affecting NF-kB p-65. In conclusion, OST inhibited TGF-b1-induced EMT, adhesion, migration, and invasion through inactivation of NF-kB-Snail pathways in A549 cells. This study provides novel molecular mechanisms for the anti-metastatic effect of OST.