Radiotherapy often accompanies with malnutrition. The incidence, the time of occurrence, severity of malnutrition and so on were varied with different types of tumor, disease stages and treatment methods. Malnutrition is not only a decrease in body weight, but also a loss in lean body mass. Lean body mass is also called fat free weight, namely the mass of body after removal of fat, which is composed of cells, extracellular water and other solid components. Muscle and bones are main components of lean body mass. Many animal experiments showed that after irradiation, the muscles would appear acute atrophy, performed by decreasing of myosin and changes in proportions of myosin heavy chain, and this phenomenon was affected by radiation doses. In the clinical studies, the decrease of muscle content after radiotherapy was also observed, which was significantly related to prognosis. Bone loss caused by radiation is also one of the important factors that lead to loss of lean body mass. In the past, it was considered to be a late adverse reaction induced by radiation. However, it was found that bone loss occurred in the early stage of radiation therapy, and it was significantly affected by the radiation dose. In summary, radiation can lead to muscle atrophy, bone loss, resulting in loss of lean body mass, which increases the risk of death and affects the quality of life of patients. Because of the effects of radiotherapy varies in different muscles and bones, in clinical settings, radiotherapists should consider the adverse effects caused by muscle atrophy and bone loss when applying radiation to important bones and muscles, thus achieving optimistic treatment effect.