Objective:To investigate the effect of metformin on proliferation and apoptosis of human anaplastic thyroid carcinoma cells,and explore the underlying mechanisms. Methods:Human anaplastic thyroid cancer cell line SW1736 was treated with different doses of metformin. MTT assay was used to detect proliferation viability of cell line. Cell cycle progression and apoptosis were analyzed by flow cytometry. Results:Metformin significantly inhibited the growth of SW1736 cells in time- and dose-dependent manner with a maximal effect at 72 h. The results of flow cytometry showed that anaplastic cancer cell cycle was arrested to G0/G1 phase by metformin. Furthermore,metformin also enhanced apoptosis of SW1736 cells in a dose-dependent manner. Compared with the control group,addition of 20 mmol/L metformin for 48 h increased the percentage of apoptotic cells from 8.3% to 13.3% in SW1736 cells. Conclusion:Metformin can inhibit proliferation and cell cycle progression and induce apoptosis of thyroid carcinoma cell line. Therefore,it may be a potential therapeutic agent for the treatment of human anaplastic thyroid carcinoma.