The author studied on the side effects of x-ray irradiation to the developing mandible of the gestation and period of grow the stage rats. For experimental observation, 100 rads, 200 rads, and 300 rads of x-ray were irradiated in regular order at the lower abdomen of the 8th day gestated rats. 5 weeks after conception, their offspring were sacrificed and their mandibles were extracted with intact form. All the extracted mandible were examined for their developing modes histological findings. The results were as followed 1) In 100-200 rads irradiated rats offsprings, their mandidles were not revealed any morphological changes except of the irregular pattern of trabeculatum. In accompany with this findings, most of all the fibroblasts and osteoclasts had their nucleus with shrunken and eccetric position. 2) In according to the increasing x-ray irradiation, marked advent of osteoclast and cortical bone resorption were observed. 3) In 300 rads irradiated rats offsprings, there irregular pattern of trabeculae and windening of bone morrow cavity in their alveolar proper.