A series of pure and aluminum (Al) doped WO3 powders were synthesized at low temperature. The as synthesized materials have been characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible (UV-VIS) spectroscopy and color difference meter, respectively. A relation was found between the color difference and the UV-VIS photochromic activity. Moreover, the optical band gap of the as-prepared powder narrowed due to the appearance of impurity levels as the Al3+ ions migrated into the WO3 crystal lattice. Some rod-like particles have been found to decrease with the increase of Al concentration. An optimum Al concentration was found to be 0.25% to present the best UV-light induced photochromic activity. The mechanism of photochromic process of the Al-doped WO3 powders has been discussed in detail based on the characterization results. (C) 2015 Elsevier B.V. All rights reserved.