The ESR spectral property of irradiated peony root was studied during development of a system for detection of irradiated medicinal plants containing crystalline sugars. The plant had a weak ESR signal near g＝2.005 before electron beam-irradiation. After 10kGy irradiation, the line shape became broader and cellulose-like signal:a pair of lines at 6mT across the center signal near g＝2.005 and sugar-like spectrum:a broader line with more than 6mT of overall spectrum width, was observed. The ESR intensity decreased considerably at 30 days post-irradiation and was stable thereafter. The stable ESR line shape of irradiated peony root resembled that of irradiated sucrose, and the intensity 30 days post-irradiation increased linearly as a function of sucrose content. This result shows that the radical concentration correlation to sucrose content in irradiated plant material is predictable.