Abstract: Human hepatoma SMMC-7721 cells,HeLa cells and human melanoma A375 cells are irradiated with ~(60)Co γ rays,and the survival fractions of the three cell lines are studied by colony-formation efficience.D_(50)(the dose of radiation necessary to reduce the survival fraction to 50%)and α/β are obtained from dose survival curve.The dose survival curves of the three cell lines show that in the range of 0～6 Gy,the survival fraction of SMMC-7 721 cells is the highest while that of A375 cells is the lowest.The D_(50) values are 2.3 Gy for SMMC-7721 cells,1.9 Gy for HeLa cells and 1.2 Gy for A375 cells,and the α/β values are 0.36,2.35 and 5.6,respectively.The results show that among the three tumor cell ldose-survival ines,SMMC-7 721 cells have the highest repair ability and the lowest radiosensitivity,while A375 cells have the lowest repair ability and the highest radiosensitivity.