In this paper the expression changes of genes involved in DNA repair induced by radiation at different doses and post-irradaition time points has been investigated and the possibility of combining some genes as biomarkers for bio-dosimeter has been evaluated. The whole body of experimental mice was irradiated with 0, 2, 4, 6 and 8 Gy ,137,Cs γ-rays at 0.80 Gy/min and 1.60 Gy/min, respectively. Peripheral blood was collected at immediate, 4, 8, 12, 24 and 48 h after irradiation and RNA was extracted by AxyPrep Kit. The mRNA transcriptional expression levels of ATM, CDKN1A, DDB2 and GADD45A were detected by real-time quantitative polymerase chain reaction (RT-PCR). Expression changes of mRNA were compared with that of unirradiated control group at the dose rate of 0.8 Gy/min. A series of dose-and time-response of the changes were figured out. A heavy change in magnitude of genes transcription induced by irradiation at higher exposure dose rate could be found. The information from gene expression analysis as a greatly potential biomarkers may be used to indicate exposure doses at different post irradiation time points in a certain dose range.