Vincristine is extensively used chemotherapeutic medicine to treat leukemia. However, it remains a critical clinical problem with regard to its toxicity and drug-resistance. AMP-activated protein kinase (AMPK) is an energy sensor that is pivotal in maintaining cell metabolic homeostasis. It is reported that AMPK is involved in vincristine-induced apoptosis. However, whether AMPK is involved in chemotherapy-resistance is largely unclear. It is well-documented that metformin, a widely used medicine to treat type II diabetes, possesses anti-cancer activities, yet whether metformin affects leukemia cell viability via vincristine is unknown. In this study, we showed that both AMPK alpha 1 mRNA and phosphorylated AMPK protein levels were significantly decreased in clinical leukemia samples. We further demonstrated that metformin sensitized leukemia cells to vincristine-induced apoptosis in an AMPK-dependent manner. In addition, knockdown of AMPK alpha 1 significantly reduced the effects of metformin on vincristine-induced apoptosis. Taken together, these results indicate that AMPK activation is critical in metformin effects on vincristine-induced apoptosis and suggest a putative strategy of a combination therapy using metformin and vincristine in treatment of leukemia.