Cochinchina momordica seeds are a kind of traditional Chinese herb. In this study, anticancer activity and underlying mechanisms were investigated with an extract using human breast cancer MDA-MB-231 cells. The survival rate was reduced in a concentration- and time-dependent manner as assessed by MTT assay. After incubation for 48 h, typical apoptotic morphological changes were observed by Hoechst 33258 dye assay. Flow cytometry revealed that the treatment obviously induced G2/M arrest and apoptosis in MDA-MB-231 cells. Furthermore, western blotting demonstrated downregulation of protein expression of PI3K, Akt, NF-κB, Bcl-2, Cdk1 and cyclin B1, whereas Bax and caspase-3 were upregulated. Our results suggest that the extract induced cell cycle G2/M arrest and apoptosis in MDA-MB-231 cells by decreasing PI3K/Akt pathway. Therefore, we propose that ECMS has potential as a breast cancer chemotherapeutic agent.