Tungsten oxide (WO3) films were deposited by DC magnetron sputtering of tungsten target in O-2/Ar atmosphere. The structures of the films following the various 02 flow rate in a fixed Ar gas supply were investigated by X-ray diffraction patterns, Raman spectra and transmission electron microscopy. The electrochromic properties were characterized by a cyclic voltammetry and UV-vis absorption spectra. The results show that nanocrystalline WO3 film with crystallite size about 10 - 20 nm, deposited at 16 Seem 02, has larger charge capacity and coloration efficiency than the other amorphous films. Post-annealing the film at 200 degrees C would create 30 - 50 nm nanocrystalline film, whose electrochromic properties are promoted further due to even larger internal volume, essential to conduct ions and electrons for electrochromic intercalation. However, the electrochromic property deteriorates apparently in the film with 60 -100 nm nanocrystallites annealed at 300 degrees C, which may be caused by another electrochromism occurring always in the well-crystallized WO3 films (c) 2005 Elsevier B.V. All rights reserved.