α-Mangostin is a dietary xanthone that has been shown to have anti-cancer and anti-proliferative properties in various types of human cancer cells. This study investigates the molecular mechanism of the apoptosis-inducing effects of α-mangostin on human hepatocellular carcinoma (HCC) cells. We observed that α-mangostin reduces the viability of HCC cells in a dose- and time-dependent manner. α-Mangostin mediated apoptosis of SK-Hep-1 cells is accompanied by nuclear chromatin condensation and cell cycle arrest in the sub-G1 phases as well as phosphatidylserine exposure. Furthermore, α-mangostin triggered the mitochondrial caspase apoptotic pathway, as indicated by the loss of mitochondrial membrane potential, the release of cytochrome c from mitochondria, and the regulation of B cell lymphoma 2 family member expression. Moreover, α-mangostin inhibited a sustained activation of p38 mitogen-activated protein kinase (MAPK) phosphorylation, and treatment with a p38 MAPK inhibitor enhanced α-mangostin-induced caspase activation and apoptosis in SK-Hep-1 cells. In vivo xenograft mice experiments revealed that α-mangostin significantly reduced tumor growth and weight in mice inoculated with SK-Hep-1 cells. These findings demonstrate that α-mangostin induces mitochondria-mediated apoptosis through inactivation of the p38 MAPK signaling pathway and that α-mangostin inhibits the in vivo tumor growth of SK-Hep-1 xenograft mice.