Electrical conductivity of fluorite phases in the system Y2O3-Ta2O5-MgO has been studied. Y0.8Ta0.2O1.7 fluorite phase, having 15 mol% oxygen ion vacancies, showed the maximum electrical conductivity. Electrical conductivity of Y0.8Ta0.2O1.7 fluorite phase increased linearly with P1/2HO due to the contribution of protonic conduction in water vapour. The electrical conductivity of the fluorite phase increased more remarkably at lower temperature, 700°C, than at higher temperatures, 800°C and 900°C. The obtained results imply that the protonic conduction greatly contributes to the electrical conductivity of the fluorite phases in the system Y2O3-Ta2O5-MgO in water vapour.