Multiple pathways are involved in maintaining genetic integrity of cell after exposure to ionizing radiations. Among them, cell cycle modulation is the most important factor of radiosensitivity. DNA-strand breaks induced by the radiations can ultimately activate either temporary checkpoint that permit time for DNA damage repair or irreversible cell cycle arrest that results in cell death. The proper cell cycle arrest can offer time for DNA repair, and the modulated mechanisms therefore may be of help for cell survival in response to ionizing radiations. Further investigations in these fields will inevitably offer a new strategy for developing radiosensitive and radioprotective drugs.