Our previous report showed that 13(E)-labd-13-ene-8α,15-diol [13(E)] isolated from Brachyglottis monroi possessed anticancer activity on human cancer cells (A549 and Hep2). In this study, we examined the inhibitory effect of 13(E) on the proliferation of human breast cancer (MDA-MB-231) cells and its anti-tumor activity in MDAMB- 231 cells xenografted mice. Also, effect of 13(E) on biochemical markers (LPO, lipid peroxidation; GSH, glutathione; ALT, alanine aminotransferase; AST, Aspartate aminotransferase) was assessed in same mice. The 13(E) was cytotoxic to the MDA-MB-231 cells. The mode of cell death induced by 13(E) was found to be apoptosis, which was judged by the morphological alteration of the cells as well as by the checking of nuclear condensation and nuclear fragmentation by 4',6-diamidino-2-phenylindole (DAPI) staining. 13(E) treatment resulted in significant decreases in tumor volume without acute side effects, including body weight loss and mortality. Biochemical parameters such as LPO, GSH, ALT and AST also significantly reverted to normal level in 13(E) treated mice (p>0.05). The results showed that 13(E) is effective in inhibiting the tumor growth in ascetic models. 13(E) has potential in the development of anti-tumor therapy.