Mutation breeding is a useful method to improve crops. More than 20 years have passed since mutation breeding with ion beams started in Japan, since which time many mutant varieties have been produced with ion beams. However, ion beams have not been sufficiently characterized in terms of mutagens for plant mutation breeding. This review introduces the characteristics of ion beams as mutagens for mutation breeding; investigated with three objectives: to obtain useful mutants with limited plant damage by irradiation treatment, the width of the mutated sector produced by irradiation, and the mutation spectrum, compared to gamma rays using rice and chrysanthemums. In addition, the optimum dose of ion beams and the criteria for an optimum irradiation dose are shown, based on which the irradiation treatment for the success of mutation breeding with ion beams is also described.