In order to observe the effect of low dose ,12,C,6+, irradiation on cell cycle and DNA damage in mouse thymus and spleen cells the mice were whole-body irradiated with 0, 10, 50, 75, 100 and 250mGy ,12,C,6+, ion and executed six hours after. Flow cytometry was used to test the changes on thymus and spleen cell cycle and the frequencies of cells with tail moment and the tail lengths were determined by the single cell gel electrophoresis. The results show that the percentages of G,0,/G,1, phase cells reduce and the percentages of S phase cells increase significantly in thymus compared with control (,p,<,0.05), while the percentages of G,0,/G,1, phase cells increase and the percentages of S phase cells decrease obviously in spleen of mice (,p,<,0.01). Similarly, the length of the tails and the number of cells with tail rise as the absorbed dose increased, which has the obviously dose-effect dependent relationship. Low dose ,12,C,6+, irradiation can effectively stimulate DNA synthesis in thymus and can be inhibited at G,0,/G,1, in spleen of mice. At the same time, it can induce DNA damage along with the absorbed doses.