Ionizing radiation has been widely used for diagnosis and treatment of diseases. Nevertheless, radiation exposure may increase the risk of skin, cardiovascular system, hematological system, and metabolic disorders, and even the risk of cancers. Rational use of ionizing radiation improves prognosis and facilitates biomedical research, while misuse or overuse may result in serious consequences. Scientific and objective assessment of health hazards of medical radiation and establishment of effective protective interventions are of great importance to control the risk of radiation exposure. This article reviews the health hazards and protective measures of medical radiation.