A number of extracts from Asian traditional medicinal herbs have been successfully used as therapeutic agents against cancers. In this study we assessed the effect of C5E on the proliferation inhibition and apoptosis of breast cancer cell lines. C5E is an ethanol extract from traditional Asian medicinal plants which have anticancer activity. Nonetheless, little is known about the underlying mechanism. Thus, we studied the mechanism of C5E-induced cell death in the human breast cancer cell line MDA-MB-231 and MCF7 cells. The cell survival rate was reduced in a concentration- and time-dependent manner, as assessed by direct cell counting. After incubation for 48 h, typical apoptotic morphological changes were observed by microscope. To determine the synergetic effect with doxorubicin, we co-treated C5E with doxorubicin in breast cancer cells, and flow cytometry revealed that co-treatment obviously enhanced sub-G1 arrest and apoptosis in MDA-MB-231 and MCF7 cells. Furthermore, we showed that pro-apoptotic marker cleaved PARP was synergistically increased with the combined treatment of doxorubicin and C5E in MDA-MB- 231, but not in MCF-7. These results suggest that the effect of combined treatment of C5E with doxorubicin on sub-G1 arrest and apoptosis in breast cancer cells could be exerted by the different mechanism and its potential use as a therapeutic agent will be helpful in treatment for breast cancer.