Radiotherapy is the most common modality for treating human cancers.Eighty percent of cancer patients need radiotherapy, either for curative or palliative purpose. Chemical modification is the concept in cancer therapy in which the state of tumor cells or normal tissues is modified such that the therapeutic gain can be achieved using conventional modalities. Radiosensitizers are the drugs that make cancer cells more sensitive to the effects of radiation therapy. Radioprotectors (also called radioprotectants) are drugs that protect normal (non cancerous) cells from the damage caused by radiation therapy. Therefore ,chemical modifier field is developing novel approaches to the cancer treatment and increasing the understanding of the basics of cancer biology.