5YSZ (5 mol. %Y2O3 stabilized ZrO2) solid electrolytes play a key role in NOx sensors for their important ion conduction performance in the sensing process. In this study, the influence of sintering and test temperature on 5YSZ solid electrolytes was investigated. The 5YSZ solid electrolyte was prepared by the tape casting process, followed by the sintering process. The ionic conductivity of the 5YSZ solid electrolyte sintered at 1450 °C is higher than that of other samples. The conductivity and activation energy of grain interior and grain boundary showed opposite tendency. The performance of ion conductivity was attributed to (1) the higher test temperature being applied to the working electrolyte and accelerated transfer efficiency and (2) enhanced diffusion of oxygen ions, which increased through the formation of dense structure.