Abstract: To investigate the effects of gambognic acid (GA) on TRAIL-induced apoptosis of cancer cells, human colon HT-29 cancer cells were treated with GA to promote apoptosis. Inhibition of the cell proliferation was measured with MTT assay and cell apoptosis was detected with formation of DNA ladders in agarose gel electrophoresis, and activation of caspase activity. The content of cytosolic reactive oxygen species (ROS) was measured with flow cytometry. The activities of Caspase-3, -8, -9 were detected using spectrophotometric assay. The levels of c-FLIP, CHOP, DR4 and DR5 in cells were tested by Western blot. Combination of GA (1 �g·mL^-1) and TRAIL (40 ng·mL^-1) significantly reduced proliferation and increased apoptosis of HT-29 cells over those induced by each agent alone. Percentage of apoptotic cells was increased to 45.5%. GA markedly enhanced the intracellular ROS generation. Expression of CHOP, DR4 and DR5 was up-regulated to 7.38, 5.41, and 4.85 times of the control group, respectively. GA promoted activation of Caspase-3, -8, and -9 by TRAIL (P < 0.05). Furthermore, the expression of anti-apoptotic protein c-FLIP was down-regulated to 0.22 ± 0.08 times of the control group. In conclusion, GA sensitizes HT-29 cells to TRAIL-induced apoptosis by promoting ROS-activated ERS pathways, up-regulating of DR4 and DR5, and inhibiting c-FLIP expression.