Recently colorectal cancer rates have increased rapidly in Taiwan. The treatment of colorectal cancer includes surgery, radiation therapy and chemotherapy. Mangosteen (Garcinia mangostana) is a famous Asian tropical fruit. γ-Mangostin is a xanthone derivative isolated from the fruit hull. In previous studies, we found evidence of anti-inflammatory and anti-brain tumor activities in γ-mangostin. In this study, we performed further studies to assess the apoptotic effects of γ-mangostin on colorectal adenocarcinoma cells HT29. γ-Mangostin showed concentration and time-dependent cytotoxic effects on HT29 cells. Microscopic observation under Giemsa staining showed that γ-mangostin induced cellular swelling and the appearance of apoptotic bodies, characteristic of apoptosis in HT29 cells. In addition, flow cytometry analysis showed an increase of hypodiploid cells in γ-mangostin-treated HT29 cells, while enhancement of intracellular peroxide production was detected in the same γ-mangostin-treated cells by DCHDA assay and DiOC6(3) staining. In view of the above results, γ-mangostin has demonstrated anticancer activity and induces apoptosis in HT29 colorectal adenocarcinoma cells. The evidence suggests that γ-mangostin could serve as a micronutrient for colon cancer prevention and is a potential lead compound for the development of anti-colon cancer agents.