Dielectric relaxation measurements for Y2O3-stabilized ZrO2 single crystals with different directions have shown anisotropic dielectric properties and approved the anisotropic diffusion paths of oxygen ions in fluorite structures suggested by neutron diffraction measurements. The activation energy including the migration energy of mobile oxygen ions obtained from the dielectric measurements suggested that oxygen ions easily diffuse along the < 100 > direction. The amount of mobile oxygen ions is also highest in the direction. The anisotropic dielectric properties are discussed along with the diffusion path of oxygen ions in fluorite structures. (c) 2007 Elsevier B.V All rights reserved.