Abstract: Abstract　Objective: To examine the effect of trastuzumab, a HER-2 inhibitor, on the nuclear transport and DNA break repair process of radiation-induced HER-2 in the breast cancer cell line BT474. The radio-sensitization mechanisms of trastuzumab were also discussed and elucidated. Methods: BT474 cells were randomly divided into two groups. Group one was treated with simple radiotherapy ( simple radiation ), whereas group two was treated with radiotherapy plus trastuzumab ( intervention ). Clone formation assay was used to observe the differences in the survival fractions between the two groups following treatments with various irradiation doses. Confocal microscopy was performed to observe the nuclear transport process of HER-2 and γH2AX expression after the radiation therapy. Western blot analysis was used to detect the HER-2 and DNA-PKcs expression in the nuclei during an early stage after radiotherapy. Results: Compared with group one, the survival fractions of BT474 cells in group two decreased significantly following radiotherapy with each irradiation dose. The results of confocal microscopy showed that the trastuzumab delayed the nuclear import process of HER-2 at an early stage post-radiation and increased the expression of γH2AX at 12 h after radiation. Western blot analysis revealed that the trastuzumab treatment decreased HER-2 and DNA-PKcs expression in the nuclei during an early stage after radiation. Conclusion: Trastuzumab can inhibit the DNA double-strand break repair in the BT474 cells during an early stage post-radiation by decreasing the HER-2 expression in the nuclei to downregulate the DNA-PKcs activity.