Abstract: Objective: To investigate the radiosensitizing effect and mechanisms of COX-2 inhibitor NS-398 on colorectal cancer cell lines HT-29. Methods: Colorectal cancer cell lines HT-29 had been incubated with 25μM NS-398 before X-ray irradiation at different doses (0, 2, 4, 6 and 8Gy). The cells were assayed for clonogenic survival to determine the radiosensitizing effect of NS-398. Flow cytometry was used to determine the effects of NS-398 and radiation on apoptosis and cell cycle distribution. The changes of mRNA and protein of Bax and Bcl-2 of HT-29 were measured by RT-PCR and flow cytometry after treatment with NS-398. Results: When survival fraction was 0.1, the sensitization-enhancement ratio was 1.76. NS-398 induced apoptosis in HT-29 cells alone and enhanced its sensitivity to irradiation-induced apoptosis. NS-398 also inhibited the proliferation of HT-29 cells. The expression of Bax mRNA and protein were increased after treatment with 25μM NS-398 at a dose-dependent manner, whereas there was no changes with Bcl-2. Conclusion: The findings indicate that the COX-2 inhibitor NS-398 has radiosensitizing effect on HT-29 cells by induction of the apoptosis and inhibition of the cell proliferation.