Abstract： Objective To compare the anti-tumor effects of low level ionizing radiation with single or fractionated irradiation method. Methods Lewis cells and S180 sarcoma were transplanted to C57BL/6 and ICR mice and were respectively irradiated with single dose(7.5 cGy) or fractionated dose(1.5 cGy×5 d) 24 h later. Then the tumor inhibitory rate, splenic NK cell activity, CD4 and CD8 levels on T lymphocyte surface, and the average longevity of tumor-bearing mice at 20 d were observed. Results The tumor inhibitory rates of Lewis cancer with single or fractionated irradiation were 40.34% and 51.19% respectively, and that of S180 sarcome were 34.26% and 30.98%. Splenic NK cell activity was elevated in 1 to 4 days after irradiation. CD4 and CD8 levels and the ratio of CD4/CD8 all increased to some different extent after irradiation. And the life prolongation rates in S180 tumor-bearing mice were 8.45% and 8.84% respectively. Conclusions Low dose and low dose rate radiation have obvious anti-tumor effects in mice with Lewis cancer and S180 sarcoma, and they enhance immunological function and prolong average longevity of the tumor-bearing mice. But there were no statistical difference(P＞0.05) in tumor growth inhibition and immunological function stimulation between the single and the fractionated irradiation.