To investigate the effect of ionizing radiation on p16 gene transcription and protein expression of thymocytes and splenocytes in mice. Northern blot was applied to determine p16mRNA level. Flowcytometry (FCM) was employed to measure p16 protein expression. The time−course experiments showed that p16mRNA level was markedly increased at4—24h in thymocytes and at4—8h in splenocytes, and p16 protein expression increased significantly at 8—48h in thymocytes (,p,<,0.05—,p,<,0.01) at 24h in splenocytes (,p,<,0.05) after 2.0Gy whole body irradiation with X−rays. The dose−effect experiments showed that p16mRNA level of thymocytes and splenocytes increased with increasing doses and the p16 protein expression increased significantly, too, when the dose increased from 1.0 to 4.0Gy for both thymocytes and splenocytes (,p,<,0.05—,p,<,0.01). p16 gene transcription and protein expression could be induced by X−rays irradiation, and tendency of the increase varied with the dose and cell type. The results suggested that p16 expression played a critical role in G,1, arrest induced by ionizing radiations