Tungsten trioxide (WO3) powders were prepared via a simple hydrothermal method. The morphology, structure and photochromic activity of the synthesized WO3 powders were studied by X-ray diffraction, scanning electron microscopy and UV-vis spectrophotometer combined with color difference meter. The results showed the synthesized WO3 powders with hexagonal phase got much better photochromic properties than the WO3 powders with cubic phase, the ones not appear until about 160 degrees C. Besides, the WO3 powder synthesized at 120 degrees C exhibited the best photochromic properties of the samples prepared below 160 degrees C, the particles of which formed a shape of clusters of cactus with uniform size and good dispersion. (C) 2011 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.