The histological and ultrastructural changes in cerebellum of mice exposed whole-body irradiation with high doses of 60Coy-ray were studied in this paper. The animals suffered from acute radiation sickness of CNS form, intermediate form and intestinal form after 16000, 8000 and 4000 rads irradiation respectively. The nuclear shrinkage of the cerebellar granular cells was the most prominent change after irradiation. The cellular necrosis was found only in individural cells. The shrinkage of nucleus, different from necrosis, with characteristic ultrastructural features, tended to recover as manifested by the appearance of nucleolus, increase of free polysomes and well development of Golgi apparatus. It was considered that the cerebellar granular cells were rather resistent to, the radiation. Some pathological changes of small blood vessels in cerebellum were also observed, but was not responsible for the degeneration of the neurons.