Objective：To observe the effects of metformin on the invasion and metastasis of undifferentiated thyroid cancer and to evaluate its underlying mechanisms. Methods：The undifferentiated thyroid cancer cell line SW1736 was treated with different concentrations of metformin. The invasive and metastatic abilities of SW1736 cells were measured by agar drop method and Transwell invasion and migration assay. Real-time PCR was used to detect the molecules mRNA involved in the process of epithelial-mesenchymal transition(EMT)，including vimentin，E-cadherin and transcription factors Snail1 and Slug. Results：Metformin inhibited the invasive and migration capacities of SW1736 cells in vitro. The inhibitory effects of metformin were dose dependent. Metformin significantly down-regulated the expressions of vimentin，Slug and Snail1 in SW1736 cells. Meanwhile，it stimulated the expression of E-cadherin，which highly expresses in epithelial cells. Conclusion：Metformin can promote the expression of E-cadherin by down-regulating the expression of Snail1 and Slug，thus inhibiting the process of EMT and decreasing the invasive and migration ability of thyroid carcinoma，which may provide a novel strategy for the treatment of thyroid neoplasms.