In this paper, dose-, dose rate- and time−effect of adaptive response of thymocyte cycle progression induced by low dose radiation were observed. Kunming mice were irradiated with different inductive doses ( ,D,1) of 25, 50, 100 and 200mGy at a fixed dose rate of 12.5mGy/min, or a fixed inductive dose (,D,1) of 75mGy with different dose rates of 6.25, 12.5, 25, 50, 100 and 200mGy/min. The mice were then irradiated with different challenging doses ( ,D,2) of 1.0, 1.5 and 2.0Gy at a fixed dose rate of 287mGy/min. The time intervals between ,D,1 and ,D,2 were 3, 6, 12, 24 and 60h. Thymocyte cycle progression was measured with flow cytometer. It has been found that with the mice which had received ,D,1 of 25, 50 and 100mGy at 12.5 mGy/min dose rate and the ,D,2 doses 6h after the ,D,1, or ,D,1 of 75mGy at dose rates of 6.25, 12.5 and 25mGy/min and the ,D,2 doses 3, 6 and 12h after the ,D,1, or ,D,2 of 1.0, 1.5 and 2.0Gy, the percentage ratio of S phase thymocytes decreased significantly (,p,<,0.05 or ,p,<,0.01), and the percentage of G,0,/G,1, and G,2,+M phase thymocytes increased significantly (,p,<,0.05 or ,p,<,0.01).too, by comparing the ,D,2 groups to the sham−irradiation group, Comparing the ,D,1+,D,2 groups to the ,D,2 groups, however, the percentage ratio of S phase thymocytes increased significantly (,p,<,0.05 or ,p,<,0.01), and the percentage of G,0,/G,1, and G,2,+M phase thymocytes decreased in varying degrees. The results suggest that irradiating the mice to 25—100mGy at dose rates of 6.25—25mGy/min 3—12h before exposing them to 1.0—2.0Gy irradiation at 287mGy/min dose rate, the adaptive response of thymocyte cycle progression could be induced under the condition of whole−body irradiation.