Deep investigations were performed for further understanding of the nanostructure of sputtered WO3 ˆlms. The as-deposited ˆlms consisted of ˆne crystallites of several nm. As the pressure increased, the ˆlm density decreased and the surface area increased owing to open pores between grains. When ˆlms were annealed at 400°C or above, they were well crystallized to form monoclinic and randomly-shaped grains. Upon this crystallization, the ˆlm shrank and its density increased slightly, while the relative surface area substantially decreased.