Disk-shaped tungsten(VI) oxide (D-WO₃) particles were synthesized according to a previously reported method consisting of pyrolysis, precipitation, and calcination, and the calcination temperature was changed (200–600 °C). The samples were characterized by field-emission scanning electron microscopy, transmission electron microscopy, powder X-ray diffraction, the Brunauer– Emmett–Teller single-point method, and diffuse reflectance spectroscopy. In addition, to evaluate the photocatalytic activities of the samples, the mineralization of acetic acid to carbon dioxide (CO₂) was measured by loading Pt particles onto the surface of the samples by photodeposition and irradiating them in an aqueous suspension with a blue light-emitting diode. Increasing the calcination temperature was associated with several changes: the crystallites grew larger, increasing the crystallinity; the specific surface area decreased, decreasing the adsorption capacity; and the rate of the photocatalytic CO₂ evolution reaction increased. Pt-loaded (0.1 wt%) D-WO₃ calcined at 600 °C showed the highest activity with a CO₂ evolution rate of 5.9 μmol h⁻¹. These results indicated that improving the crystallinity of the D-WO₃ samples was effective in increasing their photocatalytic activities.