Cordycepin (3’-deoxyadenosine) is a polyadenylation specific inhibitor, one of the components of Cordyceps militaris. It has been shown to possess many pharmacological activities including immunologically stimulating, anti-tumor, anti-virus, and anti-infection effects. However, its molecular mechanisms are poorly understood. In this study, the apoptotic effects by cordycepin were investigated in human leukemia cells. Cordycepin treatment inhibited leukemia cells growth a concentration-dependent manner by inducing apoptosis,as evidenced by morphological change and apoptotic cell death such as formation of apoptotic bodies, DNA fragmentation and increased populations of apoptoticsub G1 phase. Induction of apoptosis by cordycepin in leukemia cells were associated with modulation of Bcl-2 member and inhibitor of apoptosis (IAP) proteins expression. Cordycepin also increased ROS generation, activation of caspase-3, caspase-8, caspase-9, cleavage of poly(ADP-ribose) polymerase (PARP), -catenin and phospholipase (PLC)-1 protein. Both the this effect by cordycepin treatment were significantly inhibited by NAC, a ROS scavenger, demonstrating the important role of ROS in the observed cytotoxic effect. This results suggested that cordycepin may be a potential chemotherapeutic agent for the treatment of leukemia patients.