Allicin is an active compound derived from garlic that has been shown to have antitumor properties in vitro. The current study was designed to explore the effects and the underlying mechanism of allicin on gastric cancer cells. The MTT assay was used to detect cell viability. Transmission electron microscopy, Rh123 and propidium iodide staining, annexin V/FITC assay and the mitochondrial membrane potential were used to assess for the presence of apoptosis. Immunocytochemistry, western blot analysis, and Q-RT-PCR were used to detect gene expression. We found that allicin reduced cell viability in a dose- and time-dependent manner, partly through induction of apoptosis in gastric cancer cells. At the molecular level, allicin induced cytochrome c release from the mitochondria and increased caspase-3, -8, and -9 activation, with concomitant upregulation of bax and fas expression in the tumor cells. Allicin treatment inhibited proliferation and induced apoptosis in SGC-7901 cancer cells. Both intrinsic mitochondrial and extrinsic Fas/FasL-mediated pathways of apoptosis occur simultaneously in SGC-7901 cells following allicin treatment. Data from the current study demonstrated that allicin should be further investigated as a novel cancer preventive or therapeutic agent in control of gastric cancer, with potential uses in other tumor types.