The coagulation of Au nanoparticles dispersed in organic liquids was dramatically accelerated under the illumination of light. It was clarified through the wavelength dependence of the irradiation that particles coagulated with the illumination of visible light and coalesced upon irradiation of UV light. The coagulation process was characteristic of the surface plasmon oscillation of small metallic particles but not of the interband transition. The enhanced van der Waals attraction force or the photoinduced deionization of particles is suggested to be a cause of this finding. Particle clustering proceeded 1-dimensionally at the early stage and 1.7-dimensionally in the fractal dimension at the later stage of the coagulation. Ostwald ripening was found during the coalescence process. A UV photochemical reaction was suggested to be responsible for this process.