Microstructures of thermally evaporated WO3 thin films on sapphire substrates are investigated by wide-angle X-ray diffraction, atomic force microscope and X-ray photoelectron spectrum. The as-deposited film is amorphous and crack-free with WO3 stoichiometry on the surface. After annealing at above 400 degrees C, the film is crystallized. Compared to the monoclinic phase of the starting WO3 powder, fewer peaks are evident at room temperature for the annealed film, This highlights that the film grown on the sapphire has a preferential orientation of WO3 (200), probably because of the atomic arrangement similar to the sapphire. The crystallite sizes are estimated from the major peak to be 17.5-23.9 nm according to the Scherrer equation. The increasing of annealing temperature exhibits positive effects on the surface roughness or fractal dimension, surface area and grain size of the film. However, abnormal increments in the topographical parameters occur in case of annealing at 600 degrees C. The binding energy of the annealed film is close to that of WO3, and a small downshift of 0.1 eV reflects the formation of oxygen vacancies on the surface. The heterogeneity parameter of the film is introduced into the Schottky barrier equation. The highest sensitivity of the 500 degrees C-annealed sensor is explained in terms of the annealing temperature effect on the geometrical and chemical heterogeneities.