The work is to investigate whether P21 protein expression could be induced by ionizing radiation and the effect on cell cycle uncoupling. Immunocytochemistry assay was used to measure P21 protein expression. Polyploid was analyzed by FCM following staining of cells with propidium iodide (PI). The results showed that P21 protein expression of EL−4 cells was increased significantly 2—72h after 4.0Gy X−ray irradiation (,p,<,0.05—,p,<,0.001). In dose−effect experiments it was found that P21 protein expression also increased significantly 24 h after exposure to 0.5—6.0Gy X−rays of EL−4 cells (,p,<,0.01—,p,<,0.001). However, 8N cells were not changed in EL−4 cells irradiated to 1.0—6.0Gy by X−rays compared with sham−irradiated control (,p>,0.05). These results demonstrated that the ionizing radiation could induce P21 protein expression, but not cell cycle uncoupling. It suggested that P21 might play an important role in cell cycle uncoupling.