Antitumor effects of gallic acid on human stomach cancer KATO III cells and human colon adenocarcinoma COLO 205 cells were investigated. The exposures of KATO III and COLO 205 cells to gallic acid led to both growth inhibition and induction of apoptosis. Morphological changes showing apoptotic bodies were observed in both the cell lines treated with gallic acid. The fragmentations by gallic acid of DNA to oligonucleosomal-sized fragments, that are characteristics of apoptosis, were observed to be concentration- and time-dependent. These findings suggest that growth inhibitions by gallic acid of KATO III cells and COLO 205 cells result from the apoptosis induced by gallic acid. Thus, gallic acid might be a candidate drug for digestive gut cancer treatment to overcome the resistance to chemotherapeutic drugs.