Radiation therapy performs an important function in cancer treatment. However, resistance of tumor cells to radiation therapy still remains a serious concern, so the study of radiosensitizers has emerged as a persistent hotspot in radiation oncology. Along with the rapid advancement of nanotechnology in recent years, the potential value of nanoparticles as novel radiosensitizers has been discovered. This review summarizes the latest experimental findings both in vitro and in vivo and attempts to highlight the underlying mechanisms of response in nanoparticle radiosensitization.