Goniothalamin, a natural occurring styryl-lactone, is a novel compound with putative anticancer activities. In the present study, the mechanism of action of goniothalamin was further investigated in human breast cancer MDA-MB-231 cells. Goniothalamin treatment of cells significantly induced cell cycle arrest at G(2)/M phase and apoptosis. By means of cell cycle synchronization, the G(2)/M phase cells proved to be the most sensitive fraction to goniothalamin-induced apoptosis. Cells treated with goniothalamin revealed an increase in intracellular reactive oxygen species and a decrease in intracellular free thiol contents. The disruption of intracellular redox balance caused by goniothalamin was associated an enhancement of cdc25C degradation. Furthermore, the antioxidant N-acetylcysteine and the glutathione synthesis inhibitor dl-buthionine-(S, R)-sulfoximine, inhibited and enhanced, respectively, the effects of goniothalamin on cell cycle arrest and apoptosis. Taken together, our result demonstrates for the first time that goniothalamin disrupts intracellular redox balance and induces cdc25C degradation, which in turn causes cell cycle arrest and cell death maximally at G(2)/M phase in MDA-MB-231 cells.