Abstract: Objective To observe the effects of Herceptin on radiation induced DNA-PKcs expression and its activiation in breast cancer cell lines,and elucidate the relationship between herceptin and radiosensitization. Methods Survival fraction differentiation of breast cancer cell line from treatment herciptin was observed after X-irradiation.DNA-PKcs expression was also investigated with Western blot.Double strand break was detected by single cell electrophoretic comet assay. Results Compared with control group,survival fraction of breast cancer cell lines after radiation was significantly decreased,and expression of DNA-PKcs protein increased.Treated with herceptin survival fraction was further decreased with decreased expression of DNA-PKcs protein and activation.However,Double strand break of breast cancer cell lines was further increased. Conclusion Herceptin raised breast cancer cell lines radiosensitization.Its possible mechanism is related to DNA-PKcs expression and activation inhibition and double strand break increase.