In this study, photodissolution tests of UO2 sintered pellets were carried out in 3M nitric acid solution under UV irradiation. The light source is Hg-lamp emitting 254 wavelength. As results, chemicals such as H2O2 and NO2ㆍion were detected during photodissolution of the UO2 sintered pellets. Based on this result, a new dissolution mechanism of UO2 in nitric acid solution by photochemical reaction was suggested in this study.