Using flow cytometry (FCM), The changes in expressions of p16, CyclinD1 and CDK4 in the thymocytes of mice treated with different doses of whole—body irradiation or with a fixed dose and different post—irradiation intervals were analyzed. The results showed that the expression of p16 protein in mouse thymocytes began to increase significantly at 8h after 2Gy X irradiation, reached the peak level at 24h, kept a high level until 48h, and recovered at 72h. The CDK 4 protein expression decreased at 8h after 2Gy irradiation, reached the bottom level at 12h, and recovered at 48h. The expression of CyclinD1 protein showed a little reduction only after 2Gy irradiation. Overexpression of p16 depended on the doses of radiation (0.5—4Gy). The expression of p16 in 2Gy group was significantly increased in comparison with that in the sham—irradiated group (,p,<,0.01). The reduction of CDK4 expression was significant in 2Gy group (,p,<,0.01). CyclinD1 expression showed its decrease after 0.5—2Gy irradiation. These results suggested that p16/CyclinD1/CDK4/pRb pathway might play an important role in G,1, arrest of thymocytes induced by ionizing radiation.