The catalytic role and grain boundary effect of samaria-doped ceria (SDC) interlayers were investigated. To see the catalytic role, an SDC interlayer was deposited on a single crystalline YSZ (100) substrate by using the pulsed laser deposition (PLD) for an epitaxial film. The grain boundary effects were analyzed with SDC interlayers with various grain sizes on polycrystalline YSZ substrates. As a result, the membrane electrode assembly (MEA) with an epitaxial SDC interlayer exhibited a peak power density that was twice that of a YSZ controlled MEA. More detailed experiments were conducted with MEAs with various grain sizes. In the electrochemical analysis, the MEA with a nano grain SDC interlayer showed an increased maximum power density and lower cathodic impedance. This result suggests that the performance of low temperature solid oxide fuel cells (LT-SOFCs) could be enhanced by applying an SDC interlayer with nano grains for the enhanced oxygen reduction reaction.