To develop a simple fabrication method for a highly efficient visible-light-driven photocatalyst, we investigated the photocatalytic ability of tungsten oxide (WO3) nanoparticles prepared on substrate surfaces by the metal–organic decomposition (MOD) method. Using this method, the diameter of WO3 nanoparticles was varied from 70 to 250 nm by simply adjusting the baking temperature. The WO3 nanoparticles prepared under optimal conditions showed that their photocatalytic ability under visible light is 10 times higher than that of TiO2 fabricated by the MOD method under optimal conditions.