Abstract: Some may be malignant transformation, some may die immediately, and the other may be alive which produce radioadaptive responses and radioresistance.Recently, it has been reported that there is close relationship between cell cycle alterations and different injury effects caused by radiation.Cell cycle may be arrested and delayed by irradiation in which G₁-phase, S-phase and G₂/M-phase checkpoints may play a major role.Therefore different signal transduction pathways and moleculars are involved in the checkpoints responses.Furthermore, studies on the cell cycle checkpoints responses to radiation are quite important in radiation biology, because it could not only provide the theorical basis for the elucidation of cancer caused by radiation, but also gain further insight into investigation and developing radiosensitive drugs for clinical radiotherapy.