Ailanthus altissima has been used to settle an upset stomach, to alleviate a fever, and as an insecticide. We reported that the water extract of A. altissima induced apoptotic cell death in HL-60 human leukemia cell line. Here, we showed the dose-dependent inhibitions of cell viability by the extract, as measured by cell morphology. The cell cycle control genes are considered to play important roles in tumorigenesis. The purpose of the present study is also to investigate the effect of A. altissima on cell cycle progression and its molecular mechanism in the cells. The level of p21 protein was increased after treatment of the extract, whereas both Bcl-2 and Bax protein levels were not changed. These results suggest that A. altissima induces apoptotic cell death via p21-dependent signaling pathway in HL-60 human leukemia cell line which delete wild type p53. G1 checkpoin related gene products tested (cyclin D3, cyclin dependent kinase 4, retinoblastoma, E2F1) were decreased in their protein levels in a dose-dependent manner after treatment of the extract. Taken together, these results indicate that the increase of apoptotic cell death by A. altissima may be due to the inhibition of cell cycle in HL-60 human leukemia cell line