Nanotree-like hexagonal tungsten oxide (WO3) arrays were grown on metal tungsten substrates by a facile hydrothermal method. The WO3 nanotrees, composed of 'trunks' and 'branches', were single crystals oriented in the < 001 > direction. Nanotree thin. films exhibited efficient electrochromism due to their large tunnels in the crystal and nano-channels between the nanotrees. Moreover, their coloration efficiency and reversibility were superior to polycrystalline WO3 films. (C) 2009 Elsevier B. V. All rights reserved.