Abstract: Objective To investigate the effects of tetrandrine（Tet） on radiosensitivity of human lung adenocarcinoma SPC-A1 cells and its mechanism. Methods MTT assay was used to evaluate the cytotoxicity of Tet on SPC-A1. MTT assay were used to evaluate the inhibition ratio of irradiation group（X-ray 4 Gy）， Tet （1 μmol／L）+irradiation （X-ray 4 Gy） group and Tet group （1 μmol／L）. Cell viability after radiation was calculated by clonogenic assay. According to cell survival curve， the parameters including D0， Dq，SF2 were calculated. Flow cytometry was used to detect the cell cycle distribution of irradiation group and Tet+irradiation group. Results The IC50 of Tet on SPC-A1 cells at 24， 48 and 72 h were 10.77， 5.78 and 3.89 μmol／L， respectively. The values of D0，Dq，and SF2 of Tet+irradiation group were（1.551±0.045）Gy, （0.522±0.023）Gy and 0.503±0.008, which were lower than those of irradiation group， and SER was 1.48. Flow cytometry showed that exposure of SPC-A1 cells to 4 Gy of Xrays caused G2／M arrests （P＜0.05）. The arrests were abrogated by treated with Tet（P＜0.05）. Conclusion Tet can efficiently enhance the radiosensitivity of SPC-A1 cell. The mechanism maybe abrogating the radiation-induced G2 phase arrest. It fixes the damage of DNA，and causes reproductive cell death.