The present work was carried out to examine the effect of costunolide on the growth of several cells and characteristics of U-937 human leukemia-derived cell line. Costunolide produced a potent antitumor activity in vitro dependent on concentration against several tumor cells such as P-388, L-1210 leukemia and SNU-5 stomach cancer cells. However, it showed less cytotoxicity on normal cells such as Maccaccus rheus monkey kidney cells (MA-104) up to 200 ${mu}M$ concentration. An effect of cell differentiation by costunolide was assessed by its ability to reduce nitroblue tetrazolium (NBT), and to induce phagocytosis of latex particles. In order to establish whether costunolide induces U-937 cells to differentiate toward macrophage or granulocyte, esterase activities was measured. Based on these results, we found that costunolide having cytotoxicity on U-937 human leukemia cells was explained through differentiation inducing activity.