Leptin, an adipocyte-derived cytolcine associated with obesity, has been reported to participate in carcinogenesis. Epithelial mesenchymal transition (EMT) is also considered as a key event in tumor metastasis. The aim of this study is to investigate the mechanism of leptin in the promotion of EMT leading to metastasis in A549 lung cancer cells. We investigated the effect of leptin on migration of A549 cells using wound healing and transwell assays. The incidence of EMT in A549 cells was examined by real-time PCR and immunofluorescence staining. The expression of TGF-beta in A549 cells was detected by real-time PCR, and blocking of TGF-beta in A549 cells was achieved by siRNA techniques. Additional work was performed using 100 patient samples, which included samples from 50 patients diagnosed with lung cancer and an additional 50 patients diagnosed with lung cancer with metastatic bone lesions. Leptin expression was measured using immunohistochemistry techniques. We demonstrated that leptin can effectively enhance the metastasis of human lung cancer A549 cell line using both wound healing and transwell assays. We also found the incidence of EMT in A549 cells after leptin exposure. Furthermore, we detected the expression of TGF-beta in A549 cells, which had been reported to play an important role in inducing EMT. We showed that leptin can significantly upregulate TGF-beta at both the inRNA and protein levels in A549 cells. Using siRNA to block the expression of TGF-beta in A549 cells, we confirmed the role of TGF-beta in the promotion of metastasis and induction of EMT. Furthermore, we found that in patient samples leptin was present at higher levels in samples associated with diagnosis of lung cancer bone metastases tissue than lung cancer tissue. Our results indicated that leptin promoted the metastasis of A549 human lung cancer cell lines by inducing EMT in a TGF-beta-dependent manner.