Environmental problems arise from the pollution of ground water and soil by propyzamide 3,5-dichloro-N-(3-methyl-1-butyn-3-yl)benzamide, which is a popular herbicide. To decompose propyzamide aqueous solutions containing propyzamide and TiO_2 particles was irradiated by light. The photocatalytic decomposition was accelerated when the solution temperature and pH were high. The temperature dependence was due to the adsorption processes of propyzamide on the TiO_2 particles. The decomposition was further promoted by addition of H_2O_2 because of its effective electron-trapping and generated ・OH which was available to decompose propyzamide. Although no propyzamide was detected in the solution after the irradiation time of about 20 minutes, the decomposed intermediate compounds still remained. In order to mineralize completely propyzamide, simultaneous irradiation by light and ultrasonic waves was carried out. The hybrid effect of the irradiation by light and ultrasonic waves in conjunction with H_2O_2 was first confirmed to achieve the complete mineralization of propyzamide.