In this study, we observed the catalyst/catalyst support interfaces of Polymer Electrolyte Fuel Cell (PEFC) electrocatalysts by High-Resolution TEM. As the catalyst supports, various carbon nanofibers, including VGCF (Vapor-grown carbon fibers), Platelet type carbon nanofibers and Herringbone-type carbon nanofibers, were applied. We found larger Platinum particles in the Pt/VGCF. The interfacial structure was identified to be the (111)(Pt)//(0001)(Graphite), so that its boundary is relatively stable, whereas particle growth could easily occur. On the contrary, in the Pt/Platelet and in the Pt/Herringbone, well-dispersed Platinum particles were observed, as the catalyst/catalyst support interface is rather unstable. Moreover, twin was observed in the Platinum particles supported on the Herringbone-type nanofibers. (C) 2008 Elsevier B.V. All rights reserved.