A hexagonal form of tungsten trioxide (h-WO3, particle size: 15.9-57.1nm) was found to be formed by a direct reaction between metallic tungsten powder (W,particle size: 0.45-0.59m) and 15%-30% hydrogen peroxide (H2O2) aq solution. Oxide film on the powder surface having the similar crystal structure as h-WO3 was essential for the formation, and the surface oxide film was formed by aging the powder in air at 45 degrees C, a relative humidity of 100% (P-H2O 96hPa) for 3-28days or in ambient atmosphere at room temperature for 12years. The Rietveld analysis performed in the space group P6(3)/mcm (Z=6) indicated the crystal structures were the same as those of the reported h-WO3 and that the crystallographic characteristic was as follows: a=0.74219nm, c=0.77198nm for h-WO3 from the 28-day aged powder, and a=0.74538nm, c=0.77194nm for h-WO3 from the 12-year aged powder.