The adaptive response of thymocyte apoptosis and cell cycle progression induced by 75mGy X-rays in mice were observed. The percentages of thymocyte apoptotic bodies (TAB) and cell cycle phase count were examined with flow cytometry. It was found that the percentages of TAB increased,the percentages of S phase cells decreased and the percentages of G,0,/G,1, and G,2,+M phase cells increased after whole-body irradiation with 1.5 or 2.0Gy in mice,all were significant statistically,as compared with sham-irradiation. When the mice were irradiated with 75mGy (inductive dose,D,1,) 6h before 1.5 or 2.0Gy (challenge dose, D,2,) exposure,the damage effects of D,2, on thymocyte apoptosis and cell cycle progression were obviously weaked. Low dose radiation may induce the adaptive response of thymocyte apoptosis and cell cycle progression.