Free radicals with a long lifetime were observed in the leaves of two rice cultivars (Oryza sativa L.), Sasanishiki (UVB resistant) and Norin-1 (UVB sensitive), by electron spin resonance spectroscopy. The leaves of both cultivars grown with visible light show very similar ESR spectra composed of radical 1 (R1) and radical 2 (R2), which may be attributable to P700 cation radicals in the reaction center of photosystem I, and tyrosine cation radicals in the reaction center of photosystem II, respectively. The ESR spectrum composed of R1 and R2 radicals in the leaves of Sasanishiki grown under visible light with supplemental UVB was similar to that in the plant grown without supplemental UVB. On the other hand, the amount of R2 radicals in the leaves of Norin-1 grown under visible light with supplemental UVB was significantly smaller than that in the plant grown without supplemental UVB. It is suggested that the loss of R2 radicals in Norin-1 upon UVB irradiation is related to the instability of the plant.