A vertically arrayed flake film, "flake-wall film," of monoclinic tungsten oxide (WO3) was prepared on a transparent conductive glass by controlling anisotropic crystal growth and self-arrayed growth of WO3 hydrate with a layered crystal structure. The WO3 flake-wall film exhibited superior performance for photoelectrochemical water oxidation under visible-light irradiation compared to that of a film consisting of horizontally laminated WO3 flakes. The small differences between photocurrents under front-side irradiation and back-side irradiation and between photocurrents in the absence and the presence of methanol indicate the excellent electron transport property and long photogenerated carrier lifetime in the flake-wall film. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3525624] All rights reserved.