OBJECTIVE To study the effect of X-radiation on expression of Hmsh2 and Hmlh1 in nasopharyngeal carcinoma (NPC) CNE-1 cells, and explore the mechanism of DNA mismatch repair after radiation.METHODS The cells were divided into experimental and control groups.Experimental cells were exposed to 10 Gy radiation administered as 2 Gy per fraction. Control cells were not radiated. The distribution of cells in the cell cycle was analyzed using flow cytometry. The expression of Hmsh2 and Hmlh1 was examined by RT-PCR and West blots.RESULTS The expression of Hmsh2 a in experimental cells was significantly greater compared to control cells at 0-3rd weeks and decreased at the 4th week following radiation (P＜0.01). The expression of Hmsh2 protein in experimental cells was up-regulated and significantly greater compared to control cells at the 2nd～4th weeks after radiation (P＜0.01). Therewere no significant differences in Hmlh1 a and protein expression between experimental and control cells (P ＞0.05).CONCLUSION Radiation induces Hmsh2 expression; Hmsh2 has a role in the process of DNA repair, which maybe responsible for reduction of radiosensitivity after radiation.