Electrochromic WO3 films with size controlled nanorods were successfully fabricated by a wet chemistry method. The structure and electrochromic properties of tungsten oxide nanorods and films were studied by High resolution transmission electron microscope, Scanning electron microscope, X-ray photoelectron spectroscopy, X-ray diffraction, and electrochemical methods. The average length and diameter of the monodispersed tungsten oxide nanorods increase from 38 x 3.2 nm to 89 x 4.5 nm with help from surfactants. The results show that the electrochromic response time of WO3 nanorod films are strongly dependent on the size of the nanorods. By simply changing the size of the nanorods, the response time of the nanorod films can be regulated nearly by about five times. The electrochemical impedance spectroscopy study showed that such trend can be attributed to the size effect of WO3 nanorods upon the dynamics of H ions diffusion. At the same time, all the nanorod films showed good electrochemical stability as there were no significant performance declines after 1000 cycles. The electrochromic modulations of transmittance at 633 nm were around 22%-49%. The above stated results demonstrate that, by controlling the size of WO3 nanorods, the electrochromic properties can be easily adjusted, especially the response time, indicating potential values for different response time required at different occasions. (c) 2017 Elsevier Ltd. All rights reserved.