Objective To investigate the radiation-sensitizing effects of Ku80 silencing by siRNA interference for A549 lung cancer cells. Methods The sequences of Ku80 siRNA and negative siRNA were chemically synthesized and transfected into A549 lung cancer cells by lipofectamine. RT-PCR and Western bolt analysis were used to determine Ku80 gene expression. The transfected cells in culture dishes were irradiated with X ray at doses of 2 Gy, 4 Gy, 6 Gy, 8 Gy, 10 Gy, respectively. Once all treatments were completed, the cells were processed with the colony formation assay. Results RT-PCR detection showed that Ku80 mRNA levels in A549 lung cancer cells were reduced after transfected with Ku80 siRNA at 24 h, 48 h and 72 h time points. Western blot analysis showed that Ku80 protein content decreased at 48 h and 72 h time points compared with the control group ( P lt; 0. 05 ) . Cloning formation assay indicated that radiosensitivity of A549 lung cancer cells was enhanced after transfected with Ku80 siRNA. Conclusion Ku80 siRNA can effectively inhibit Ku80 gene expression of A549 lung cancer cells, and therefore enhance its radiosensitivity.