Application of solid oxide cells to co-electrolysis of H₂O and CO₂ that uses an extra power derived from the renewable energy resource of solar or wind power has been attracting much attention since carbon dioxide can be reused into hydrocarbon fuels. In particular, when using a catalyst or high pressure at downstream, reactant gases of CO and H₂ in the co-electrolysis can be converted to synthetic hydrocarbon fuels that are possible to be stored or transported. This study introduces the method that simplified the process of upstream and downstream in a pressurized operation system for co-electrolysis and the electrochemical performance of the cells which was dependant on operation pressure. It was confirmed that the cells could be operated at higher current densities upon increasing the operation pressure as the polarization resistance dramatically decreased.