Summary 　Radiation, which represents a large influence on the human body with less energy, attacks DNA selectively. Radiation produces not only direct injury to DNA, but also indirect injury to DNA by converting the surrounding water into free radicals. Although radiation impairs the mitochondrial DNA than nuclear DNA, the influences to cell, tissue, and organ induced by nuclear DNA are larger than those induced by mitochondrial DNA. Most of radiation-damaged cells recover by themselves. Therefore, when assessing the radiation impact of the human body, we need to know the radiation exposure “dose rate” instead of “dose”. The radiosensitivity of a tissue depends on the number of undifferentiated cells in the tissue, their mitotic activity, and the length of time they are actively proliferating (Bergonié-Tribondeau law). Various types of protective agents for radiation exposure have been developed. Most practical agent is the absorption inhibitor and excretion promoting agent of radioactive isotopes.