Addition of sodium salts in hydrothermal synthesis was revealed to significantly influence the morphology and particle size of WO3 particles due to oriented growth. The plate-like shaped WO3 particles prepared with appropriate amounts of alkaline salts such as NaHCO3 and NaOH exhibited much higher activity for O-2 evolution in the presence of Fe3+ electron acceptor, compared to that prepared without any additives and a highly-active commercial one.