The ionic and electronic conductivities in ZrO2-CeO2-Y2O3 mixed conductors were successfully measured by using the electron blocking method. The observed oxygen partial pressure dependence of the ionic conductivity was well explained by the behavior of the oxygen vacancies produced from the reduction of Ce ions at low oxygen partial pressures. The observed oxygen partial pressure dependence of the electronic conductivity was well explained by the electron hopping process between Ce3+ and Ce4+ ions in the reducing conditions.