We have demonstrated that nanostructured Prussian blue (PB) film can be directly grown on fluorine-doped tin oxide (FTO)-coated glass substrate by using a facile and template-free hydrothermal technique. The as-grown film with a lot of nanochannels is well adhesive to substrate. An electrochromic device fabricated using the as grown PB film shows a fast coloration/bleaching response of 2.4/1.0 s and a high coloration efficiency of 87A cm(2) C-1, which shows a promising application in high-performance electrochromic smart windows.