Radiation therapy kills cancer cells by flood of free radicals. Radiation ionizes and/or excites water molecule and makes highly reactive species, i.e. free radicals and/or reactive oxygen species. Free radical chain reactions reach to a biological important molecule, then oxidize the molecule and finally disrupt the function of the molecule. Tissue oxygen and/or redox status, which can influence the course of the free radical chain reaction, can affect the results of radiation therapy. Previous observation of tissue oxygen and/or redox status must be helpful for planning a safe and efficient radiation therapy. Magnetic resonance based redox imaging techniques, which can give tissue redox status non-invasively, have been developed not only for previous diagnostic information but also for estimating effectiveness of the treatment. Redox imaging technique is now spotlighted to achieve radiation theranostics.