An increasing number of natural occurring chemical compounds have been identified to be potent in the treatment of various human diseases including cancer. Many of these substances are found in oriental herbs or plants. Recently, some chemopreventive extracts of herbs or plants have been shown to be anti-tumorigenic. Mixed extract of herbs or plants might contain different chemopreventive or chemotherapeutic effects. It is therefore very attractive and important to identify combination effects of mixed extract of herbs or plants. This study was designed to further investigate the anti-tumor effects of extracts of the aqueous root extract of Ailanthus altissima (REA) elucidate the potential mechanisms using an in vitro system. We found that the extracts of REA was able to induce cell cycle arrest at G1 phase and induce apoptosis in K562 cells in a time-dependent manner. During induction of apoptosis is accompanied by the cleavage of PARP and DNA fragmentation. The mechanisms mediated through down-regulation of bcl-2 protein and cleavage of caspase-3 in human leukemia HL-60 cells. Our results suggest that REA induces apoptosis in human leukaemic cell lines K562 and HL60