12-O-Acetylazedarachin B (1), isolated from the fruit extract of Melia azedarach, exhibited potent cytotoxicity against leukemia (HL-60) (IC(50) 0.016 µM) and stomach (AZ521) (IC(50) 0.035 µM) cancer cell lines. Upon assessing the apoptosis-inducing activity in HL-60 cells, compound 1 exhibited induction of apoptosis detected by the observation of membrane phospholipid exposure and DNA fragmentation in flow cytometry. Western blot analysis showed that 1 markedly reduced the levels of procaspases-3, 8, and 9, while being increased the levels of cleaved caspases-3, 8, and 9. In addition, compound 1 increased significantly Bax/Bcl-2 ratio. These results suggested that 1 induced apoptotic cell death in HL-60 via both mitochondrial and death receptor-mediated pathways. Therefore, compound 1 may be promising lead compound for developing an effective drug for treatment of leukemia. Flow cytometric analysis suggested that the cytotoxicity of 1 against AZ521 is due to inducing apoptosis as well as necrosis with the latter predominated.