Although micro-patterned membranes improve the performances of proton exchange membrane fuel cells, their intrinsic effect has not been clarified. This paper aims to clarify the effect of the patterned surfaces in terms of the overvoltage by fabricating well-defined micro-pillar structures on the cathode side of the membrane via thermal imprint lithography. Cell performance data and numerical simulation revealed that the pillar structures reduce the proton transport resistance and mass transport loss in the cathode catalyst layers. The reduced proton transport resistance expands the catalyst layers contributing the power generation toward the interface between the catalyst and gas diffusion layers. This decreases the average permeation distance of oxygen to react at the catalyst, which would explain the observed reduction of the concentration overvoltage. The pillar structures reduce the ohmic overvoltage predominantly at a relative humidity of <25% and a current density of <500 mA/cm(2). The concentration overvoltage suppression becomes more significant at higher humidity and current densities where the oxygen diffusion to the catalyst is blocked by water. Our findings are useful to design optimum pillar structures that maximize the fuel cell cathode performances. (C) 2013 The Electrochemical Society. [DOI: 10.1149/2.085304jes] All rights reserved.