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 mm wavelength. As results, chemicals such as H₂O₂ and NO₂＾- ion were detected during photodissolution of the UO₂ sintered pellets. Based on this results, a new dissolution mechanism of UO₂ in nitric acid solution by photochemical reaction was suggested in this study.