Uterine leiomyomas are the most common gynecological benign tumors and greatly affect reproductive health and wellbeing. Metformin is the most widely used antidiabetic drug in the world, and there is increasing evidence of a potential efficacy of this agent as an anticancer drug. In order to understand metformin's anti-tumorigenic potential better, in this study, we investigated the inhibitory effect of metformin and expression of key targets of metformin cell signaling in leiomyoma cells. Cell proliferation was assessed after exposure to metformin. Apoptosis was assessed by western blotting for cleaved-PARP and TUNEL staining. The expressions of phosphorylated AMPK and phosphorylated S6 were determined by western blotting. Metformin potently inhibited ELT-3 cell proliferation in a dose-dependent manner. Western blotting analysis demonstrated that metformin induced phosphorylation of AMPK and the inhibitory effect was attenuated with AMPK inhibitor, compound C. In parallel, treatment with metformin decreased phosphorylation of S6 protein. These experimental findings show that metformin is a potent inhibitor of cell proliferation in leiomyoma cells. This effect is mediated by AMPK activation and subsequent inhibition of the mTOR pathway. Thus, this study provides a possible mechanism of the action of metformin in the inhibition of leiomyoma cell growth.