Abstract: The exposure of mammalian cells to X rays results in the prolongation of the cell cycle,including the delay or the arrest in G₁,S and G₂ phase.The major function of G₁ arrest may be to eliminate the cells containing DNA damage and only occurs in the cells with wild type p53 function whereas G₂ arrest following ionizing radiation has been shown to be important in protecting the cells from death and occurs in all cells regardless of p53 status.So the study on G₂ phase arrest of the cell cycle induced by ionizing radiation has currently become a focus at radiobiological fields.