Radiotherapy is the most common cancer treatment modality applying ionizing radiation. In majority of the cases, RT necessarily affects on normal tissues around the tumor. Silymarin is one of the most important chemical ingredients of the milk thistle. Silymarin composed of six flavonolignans that Silibinin, with 40-50%, is the most one. In view of the innumerable beneficial effects of silymarin in the management of ROS-mediated diseases, it was felt that silymarin could act as a promising radiation countermeasure agent. In this review paper we introduce Silymarin as herbal radioprotectors.