We synthesized various cuboid-type tungsten trioxide (WO3) particles using a hydrothermal method followed by calcination at various temperatures between 200 and 500 degrees C. Visible-light photocatalysis of these particles was investigated for oxygen (O-2) evolution from water in the presence of sacrificial silver(I) sulfate. The calculated crystallite diameters increased with increasing calcination temperature, suggesting that the level of crystallinity increased with calcination temperature. The average specific surface area of the particles was almost the same for all calcination temperatures in the range 200-400 degrees C, but was decreased at 500 degrees C. The photocatalytic reaction rate of O-2 formation increased with increasing calcination temperature up to 400 degrees C, and then decreased as calcination temperature was further increased to 500 degrees C. We therefore concluded that the rate increased with calcination temperature up to 400 degrees C due to the increasing crystallinity, but then decreased due to the decreased specific surface area.