Nanostructural design of catalyst layers on Nafion 117 membrane was carried out by electrophoretic deposition (EPD) method. An ethanol suspension containing catalyst composite particles of Vulcan XC72 fine carbon-supported Pt catalyst and Nafion ionomer was utilized. The catalyst layers prepared were well attached to both sides of the membrane without mechanical pressing and heating processes, and their thicknesses were easily controlled in a submicrometer scale by the EPD duration and applied electric field intensity. As well, Nafion loading in the catalyst layer was controllable by Nafion content in the suspension. We applied transmission electron microscopy to characterize catalyst layers and revealed that those prepared by EPD method had porous structures compared to the hot-pressed one. These structures were favorable for fuel gas diffusion and formation of triple-phase boundaries, resulting in improvement of Pt utilization up to 76%. Accordingly, the membrane electrode assembly prepared by EPD method exhibited higher cell performance with less than half the amount of Pt loading compared with the hot-pressed membrane electrode assembly. (c) 2007 The Electrochemical Society.