Hexagonal structured WO3 films with tree-like morphology were synthesized on tungsten foils by a hydrothermal method. Each nanotree was composed of several (typically six) nanosheet-shaped "branches". TEM examination revealed that the nanosheet was a single crystal and its long axis was oriented toward < 0 0 1 > direction. The WO3 nanotree films retain the hexagonal structure after being annealed up to 400 degrees C for 2 h, while they have a phase transition to monoclinic structure after being annealed at 500 degrees C for 2 h. Electrochromic (EC) performance of the films was examined in a propylene carbonate solution of 1 M LiClO4 using an electrochemical workstation and an UV-Vis spectrometer. Due to the large tunnels of hexagonal structure and highly porous surface morphology, a large modulation of optical reflectance of WO3 nanotree films up to 30% and coloration efficiency of 43.6 cm(2) C-1 at 500 nm were achieved by annealing the WO3 nanotree films at 400 degrees C for 2 h. (C) 2011 Elsevier B.V. All rights reserved.