Single cell microgel electrophoresis(SCGE) and MTT assay were used to determine the relationships between radiation—induced initial DNA double—strand breaks(DSBs), the ability of cells to repair DSBs and cellular radiosensitivity in both AT5BIVA and GM637 cell lines. The results demonstrated that AT5BIVA cell line was significantly more radiosensitive than GM637. The initial DSBs in both AT5BIVA and GM637 lines increased with the rise of irradiation dose,and showed significant dependence on dose. At a given dose of 20Gy, radiation—induced initial DSBs in AT5BIVA cell line was signficantly higher than those in GM637. The ability of AT5BIVA cell line to repair DSBs was more powerful than that of GM637. The results suggested that the radiation—induced initial DSBs in cells and the ability of cells to repair DSBs both correlated with cellular radiosensitivity to a certain extent, and could be used as potential predictors of intrinsic radiosensitivity of cells.