Gleditsiae Semen (GS) has been used in both Korea and China as herbal medicine for the treatment of cephalalgia, catharsis, and other diseases. However, the apoptosis of GS against human cancer cells has not previously been investigated. The primary objective of this study was to determine the mechanisms inherent in GS-induced cytotoxicity and apoptosis, using methanolic extract of GS (GSE) in HT-29 human colon carcinoma cells. We found that GSE induced cytotoxicity in HT-29 cells in a dose-dependent manner, and this effect was verified via a lactate dehydrogenase release assay and a colony formation assay. In particular, HT-29 cells showed extensive cell death when treated with $50;{mu}g/mL$ of GSE; the calculated $IC_{50}$ value was $20;{mu}g/mL$. It induced characteristic apoptotic signs in HT-29 cells, including chromatin condensation and DNA fragmentation, occurring within 6-24 hr when the cells were treated at a concentration of $50;{mu}g/mL$. Interestingly, we detected the activation of caspase-3 and -9, but not caspase-8, and apoptotic bodies in GSE-treated HT-29 cells. Collectively, our results indicate that GSE induces apoptosis via a mitochondria-mediated apoptotic pathway, and these findings may be significant with regard to the development of a new drug for the treatment of human colon carcinoma cells.