Abstract: Disclosed is a composition for improving radiotherapy for cancer, containing tetra-arsenic oxide (As4O6) as an active ingredient. The disclosed composition for improving radiotherapy for cancer improves the efficiency of radiotherapy, and thus reduces side effects caused by the high-dose radiation. The disclosed composition uses tetra-arsenic oxide, the safety of which is proven, from among conventional arsenic compound derivatives, to prevent side effects caused by arsenic trioxide, taxol, cisplatin, fluorouracil, leuprolide, or the like, which are known as conventional radiosensitizers.