Summary 　As the initial step of radiation effects on biological systems, ionizing radiations deposit their energy to induce excitation and/or ionization of water molecules. The resultant free radicals such as hydroxyl radical and reactive species cause oxidative modifications of biological components such as DNA and lipids. Therefore, a compound that scavenges free radicals is potentially a radioprotector, which is effective only present before irradiation. Another type of compounds that affect immediately after irradiation is possible. This type of compounds is called mitigator. Several examples of protector and mitigator were shown with an emphasis on our own works.