The oxygen fugacity (fO2) may affect the ionic conductivity of olivine under upper mantle conditions because Mg vacancies can be produced in the crystal structure by the oxidization of iron from Fe2+ to Fe3+.Here we investigated olivine ionic conductivity at 4 GPa,as a function of temperature,crystallo-graphic orientation,and oxygen fugacity,corresponding to the topmost asthenospheric conditions.The results demonstrate that the ionic conductivity is insensitive to fO2 under relatively reduced conditions(fO2 below Re-ReO2 buffer),whereas it has a clear fO2-dependence under relatively oxidized conditions(fO2 around the magnetite-hematite buffer).The ionic conduction in olivine may contribute significantly to the conductivity anomaly in the topmost asthenosphere especially at relatively oxidized conditions.