The ICR pregnant mice were irradiated at 1.5Gy in every 6 hours in the period of organogenesis in order to classify the stage specificity of the embryonic effects of radiation and the stage of development differentiation of the primordium of each major organ. Intrauterine death, fetal body weight and external malformation in live fetuses were observed on day 18 of gestation. There was no statistically significant difference in the intrauterine mortality at any stage organogenesis. The fetal body weight of the mice irradiated in the intermediate stage of organogenesis showed significantly lower. There were specific highly sensitive stages in the incidences of each external malformation, that is exencephalia, open eyelid, cleft palate, anomalies of extremities and anomalies of the tail. At these stage, the primordia of the major organs are established in ICR mice.