A solid oxide fuel cell (SOFC) and a solid oxide electrolysis cell (SOEC) have been investigated using a zirconia- or ceria-based solid electrolyte. The electrode polarization strongly depended on the operation condition of SOFC and SOEC. Ni-YSZ cermet and Sr-doped LaMnO3 resulted in small hydrogen and oxygen electrode polarizations in the SOFC operation whereas Pt and Sr-doped LaCoO3 electrodes were preferable for the SOEC condition. Significant concentration overvoltage was observed in the V-I characteristics of SOFC and SOEC operated in different P-H2 and P-H2O atmospheres. Although the use of ceria as an electrolyte is effective in lowering the electrode polarization, high applied voltage leads to reduction of ceria electrolyte due to electronic conduction by the reaction of Ce4+ to Ce3+.