Diffusion of oxide ions in yttria-stabilized zirconia at high temperature was investigated by means of molecular dynamics simulation. Variation of the diffusion coefficient was examined in wide ranges of composition and temperature. The activation energy of the oxide ion diffusion was found to increase with increasing yttria content. The value of the activation energy was consistent with the experimental result using carefully prepared single-phase specimens. Temperature and compositional variations of the diffusion coefficient were interpreted by both the activation energy and the amount of oxygen vacancies.