To survey the variance of Gadd45 mRNA in different people’s blood exposured to ionizing radiation, which is necessary in evaluating the feasibility of Gadd45 as a radiation biodosimeter, 27 normal persons’ peripheral blood was irradiated with ,60,Co γ-rays in vitro. And its RNA was isolated from white cell. The relative real-time quantitative PCR by standard curve method was set up using β-actin as internal standards. With this method, Gadd45 mRNA expression change was observed after ionizing radiation. It has been found that in 0~2.5Gy range the relationship between the ratio of Gadd45 mRNA / β-actin and irradiation dose represents certain line correlation. A linear regression equation (,y,=0.216+0.258,x,p,<,0.001,R,2,=0.650) has been obtained using statistics regression analysis method which can be used to estimate the biological doses involved in radiation accidents though the variance of the Gadd45 mRNA level among different people is marked.