Cholangiocarcinoma (CCA) is the most common biliary malignancy and the second most common hepatic malignancy after hepatocellular carcinoma (HCC). Treatment with the anti-diabetic drug metformin has been associated with reduced cancer incidence in patients with type 2 diabetes. Thus, the present study evaluated the effects of metformin on human CCA cell proliferation in vitro and in vivo and identified the microRNAs associated with its antitumor effects. Metformin inhibited the proliferation of the CCA cell lines HuCCT-1 and TFK-1 and blocked the G0 to G1 cell cycle transition, accompanied by AMP kinase pathway activation. Metformin treatment also led to marked decreases in cyclin D1 and cyclin-dependent kinase (Cdk) 4 protein levels and retinoblastoma protein phosphorylation. However, this drug did not affect p27kip protein expression. In addition, it reduced the phosphorylation of Axl, EphA10, ALK and PYK, as well as tumor proliferation in athymic nude mice with xenograft tumors. Furthermore, it markedly altered microRNA expression. These findings suggest that metformin may have clinical use in the treatment of CCA.