The present work looked at the co-doping effect of the typical trivalent metal oxides on the electrochemical performance of 8 mol% of MgO stabilized ZrO2 (Mg-PSZ) electrolyte ceramics used for oxygen sensors. An appreciable increase in the ionic conductivity occurred on substituting MgO with trivalent metal oxides except La2O3 in the Mg-PSZ electrolyte ceramics across the measured temperature range. The conductivity was related to the co-dopant ionic radius and the ionic conductivity increased with the co-dopant ionic radius of the trivalent metal oxides, reached a maximum at about 1.02 Å, and thereafter decreased. Through comparison and analysis, the Y2O3 exhibited excellent superiority than other trivalent metal oxides and the oxygen sensor assembled by 1Y7Mg-PSZ electrolyte possessed more excellent response speed and stability.