Gambogic acid (GA), an ingredient isolated from Garcinia hanburyi, has potent anticancer activity both in vitro and in vivo. In the present study, we examined the effects of GA on intracellular microtubules and reconstituted microtubules in vitro. Immunofluorescence microscopy revealed that 2.5 muM GA caused microtubule cytoskeleton disruption and microtubule depolymerization in human breast carcinoma MCF-7 cells, thereby reducing the amount of polymer form of tubulin and increasing the amount of monomer form of tubulin. We further confirmed that GA could depolymerize microtubule associated protein (MAP)-free microtubules and MAP-rich microtubules in vitro. Thus we suggested that GA-induced G2/M phase cell cycle arrest may be attributed to its depolymerization of microtubules. We also revealed that phosphorylation levels of p38 and c-Jun N-terminal kinase-1 (JNK-1) were increased markedly by GA, resulting in apoptosis of MCF-7 cells. Taken together, our results suggested that GA depolymerized microtubules and elevated the phosphorylation levels of JNK1 and p38, which caused G2/M cell cycle arrest and apoptosis in MCF-7 cells.