Malignant tumors are tremendous heath problems facing by the medical world. In order to achieve the purpose of curing malignant tumor, numerous therapeutic strategies have been developed. Radiotherapy is one of the main therapeutic strategies for malignant tumors. Current imaging strategies cannot display exact infiltrating margins, radio-resistance generated by irradiated tissue, and intercurrent damage to healthy tissues during radiotherapy. Therefore, novel strategies to solve these problems are urgently needed. Nanomaterials have specific physical and biological properties that can help clinician to distinguish margins of infiltrating tumors as a novel contrast agent. Besides, nanoparticles can significantly enhance the effect of radiotherapy by generating reactive oxygen species (ROS) or influence cell cycle. In addition, nanomaterials can also help in diminishing the intercurrent damage caused by radiotherapy. So nanomaterials have very promising prospect in the radiotherapy of malignant tumors. This review mainly focuses on the applications of nanomaterials in radiotherapy for malignant tumors; especially it applies to lesion imaging and their radiosensitizing effects.