A novel eudesmane-type sesquiterpene compound, 1-oxoeudesm-11(13)-eno-12,8a-lactone (OEL), was isolated from Aster himalaicus. Its effect on apoptosis in human breast adenocarcinoma (MCF-7) cells was investigated. MTT assay showed that OEL substantially reduced the viability of KB, MCF-7, U87, A172, and MG-63 cells. MCF-7 cells were used to further evaluate the antitumor effects and anticancer mechanisms of OEL. OEL-induced apoptosis was characterized by chromatin condensation, formation of apoptotic bodies, and phosphatidylserine on extracellular surface; these effects were confirmed by DAPI nuclear staining and flow cytometry. Increased expression of Bax and deceased expression of Bcl-2 were also observed in OEL-treated MCF-7 cells. Moreover, OEL induced the loss of mitochondria membrane potential, release of cytochrome C, activation of caspase-9, and generation of reactive oxygen species. These findings indicate that reactive oxygen species generation and mitochondria activation were involved in apoptosis induced by OEL in MCF-7 cells. The results from our study demonstrated that OEL may be a promising pro-apoptotic compound that could be used to develop novel anticancer drugs.