Although momilactone B has been studied as an allelochemical of rice (Oryza sativa L.), to date we have no report showing the effect of momilactone B on mammalian cells. This study was undertaken to examine whether this allelochemical has anticancer activity on cancer cells. We show here that momilactone B at micromolar doses has antitumor efficacy by inducing apoptosis in several blood cancer cells including human leukemic T cells. In addition, our study elucidated that anticancer activity of momilactone B on human leukemic T cells resulted from the induction of apoptosis via caspase and mitochondria. From these results, momilactone B can be considered as a novel therapeutic strategy for human leukemic T cells from its direct apoptosis-inducing activity.