Access to clean water is necessary for the well-being and economic development of society. Thus, water contamination from human activity such as industry is a serious threat. Recently, the method in which organic contaminants in water are decomposed by photocatalysis has been proposed. However, the rate of decomposition with photocatalysis alone is slow. To enhance the degradation, the authors have developed a process that uses photocatalytic TiO2 in combination with ultraviolet (UV) light and high-frequency ultrasound (US). High-frequency US irradiation generates mist that contains TiO2 to increase the reaction area and to overcome the limited light penetration depth. To clarify the effect, the decomposition rate of phenol, known as a representative organic compound in wastewater, is experimentally evaluated. The results support that the proposed process enhances phenol degradation. This technology may be applied to decompose organic gaseous pollutants in waste gas.