To elucidate the role of p53 and its down-stream gene in G,1, arrest of mouse thymocytes induced by ionizing radiation, Flow cytometry (FCM) and northern blot were used to analyze the changes of cell cycle, expression of p53 and its down-stream gene of thymocytes of Kunming mice after whole body irradiation (WBI) with X-rays. The results indicated: (1) The number of G,1, phase cells of thymocytes began to significantly increased at 4h after WBI with 2Gy X-rays, and reached the peak level at 24h, and kept a higher level until at least 48h. (2)The percentages of p53 + cells of mouse thymocytes increased from 1h to 8h, and p21+ from 4h to 8h after 2Gy WBI. However, no change was found in the percentage of GADD45 + cells of mouse thymocytes. (3) WAF1 mRNA level began to increase at 1h, and reached the peak level at 2h, and kept a higher level till to 24h, and recovered to sham-irradiation level at 48h after WBI. There was no change of GADD45 mRNA level, in accordance with its protein level change. These results suggested that p53-WAF1 pathway may be play an important role in the G,1, arrest of mouse thymocytes induced by ionizing radiation.