Platinum nano-dendrites of various sizes were applied as cathode catalysts in a proton exchange membrane fuel cell (PEMFC). The membrane electrode assembly (MEA) fabricated with Pt nano-dendrites showed very high ORR activity at high potential ranges due to lower activation overpotential. But it showed the cell performance only comparable to commercial Pt/C due to Ohmic and mass transfer resistance at high current density ranges. These results demonstrate that both high intrinsic activity of the catalysts and the formation of three-phase interface with efficient proton, electron, and mass transfer should be considered together when shape-controlled metal nanoparticles are used as cathode catalysts in a PEMFC.