Objective To analyze the effect of high and low dose radiation on the expressions of Th1,Th2 and Th3 /Tr1 related-genes in mice thymocytes and investigate the possible underlying molecular mechanism.Methods ICR mice were randomly divided into low-dose group (0.075 Gy),high-dose group (2.0 Gy) and sham-control group.The mouse thymus tissue was extracted at 16 hours after irradiation and the expressions of Th1-Th2-Th3 related genes were measured by PCR array.Results Eight genes were up-regulated and five genes were down-regulated after low dose radiation (0.075 Gy);while 54 genes were up-regulated and three genes were down-regulated after high dose (2.0 Gy) radiation.These genes included Th1 cell related genes,Th2 cell related genes,Th3/Tr1 cell related genes,Th1/Th2 immune response genes and transcription factor related genes.Low dose radiation induced up-regulation of Stat4 and Socs1 of genes related to the Th1 cells,and it induced down-regulation of IL-4ra,Cebpb,Gata3 and Tgfb3 associated with Th2 and Th3 cells,which lead to Sftpd genes up-regulation of Th1 immune response eventually.The high dose radiation up-regulated all of Th1,Th2 and Th3/Tr related genes and also enhanced the expressions of Cd86,IL-18,IL-10 and Irf4 genes related to Th2 immune response,but it did not alter the gene expression of Th1 immune response.Conclusions Low-dose radiation induces Th1-type immune response,while high doses radiation triggers Th2 type immune response.