Recent studies both in vitro and in vivo of G. procumbens exhibits chemopreventive properties for tumor inhibition on several types of cancer. Our study was carried out to observe the anticancer property of ethyl acetate fraction of G. procumbens leaves (FEG) on breast cancer cells as well as the co-chemotherapeutic potential, and to investigate its molecular mechanisms. 3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay was used to measure the growth inhibitory effect of FEG, doxorubicin (DOX), and 5-fluorouracil (5-FU) and their combination. Flowcytometry, 4′,6-diamidino-2-phenylindole (DAPI) staining, and immunobloting were used to explore the mechanism of cell cycle arrest and apoptosis. FEG inhibited cell proliferation, induced G 1 phase arrest and apoptosis. The inhibitory effect of FEG was enhanced when combined with Dox and 5-FU. The apoptosis induction was related to the increase of c-PARP expression after combination treatment of FEG and Dox or 5-FU on MCF-7 cells. However, treatment of DOX, 5-FU, and FEG on T47D cells, resulting no significance DNA fragmentation and nuclei condensation evidance. Only combination treatment of 5-FU + FEG showed c-PARP expression in T47D cells. In T47D cells, The FEG treatment also caused the decrease of microtubule expression as shown by Western blotting assay. The decreasing level of microtubul expression might be caused by protein aggregation, as shown by immunostaning using α-tubulin antibody. All these results suggest that FEG potentiates the DOX and 5-FU efficacy on MCF-7 and T47D cells. FEG induces T47D cell death through different mechanism than MCF-7 that proposed to be mitotic catastrophe. The FEG may have specific targeted on microtubule integrity modulation leading to the cell cycle arrest and proliferation inhibition. Further FEG could be developed as a co-chemotherapeutic agent for reducing side effect and have specific molecular target for breast cancer. © 2012 Institute of Oriental Medicine, Kyung Hee University.