The growth inhibitory effects of black tea polyphenol, theaflavin (TF-1) and its peracetylated derivative (ATF-1) in human leukemia cancer cells were examined. TF-1 and ATF-1 displayed strong growth inhibitory effects against human leukemia HL-60 cells. ATF-1 was more potent against the growth of HL-60 cells and induction of apoptosis through modulation of mitochondrial functions regulated by reactive oxygen species (ROS). ROS generation occurs in the early stages of ATF-1-induced apoptosis, preceding cytochrome c release, caspase activation, and DNA fragmentation. The molecular mechanism of ATF-1-induced apoptosis was also investigated.