Tungsten trioxide (WO3) films with thickness of 0.9-6.7 mum have been deposited on glass-slide substrates, using rf magnetron sputtering in an atmosphere of mixture 80% Ar and 20% O-2. The as-deposited films had a dark metallic color, like the W target, at a working gas pressure P-W of 1 mTorr. Yellow films,resulted at a P-W of 3 mTorr. With a further increase of Pw, the film color changed to pale yellow. From the x-ray diffraction patterns, the as-deposited films were polycrystalline crystallizing in the monoclinic crystal structure with high c-axis orientation perpendicular to the film plane. The optical transmittance of the films deposited at a P-W of 1 mTorr is nearly zero. However, the transmittance of the films deposited at other P-W are larger than 70% in the wavelength, lambda, ranging from 500 to 900 nm. With decreasing X to 400 nm, the transmittance decreases steeply to zero. The lambda at this absorption edge is longer than that in TiO2 and comes in the visible region. The surface morphology of the films depends on P-W. This different morphology may be attributed to the effect of the substrate heating by plasma emission because of the high plasma density at higher P-W. The morphology of the films may also depend on the crystallinity of the WO3 films. As Pw increased, the surfaces of the films became rougher but the grain sizes of the films did not always become larger. The WO3 films deposited in this study may be used for the underlayer of TiO2 photocatalyst. (C) 2003 American Vacuum Society.