Oxygen diffusion in the oxygen ion conductor of gadolinia-doped ceria (CeO2)(1 - x)(Gd2O3)(x/2) was investigated by means of the molecular dynamics simulation. The atomic arrangement with Gd-Gd pairs in gadolinia-doped ceria was found to be energetically more stable and more realistic than that with isolated Gd atoms from the calculation of the enthalpy of formation and the lattice parameter. The compositional dependence of the diffusion constant was in good agreement with the experimental one obtained from the data of electrical conductivity. The Gd-O nearest neighbor distance was longer than that of Ce-O and they were decreased with the increase of the gadolinia content in an excellent agreement with an EXAFS study. The oxygen coordination number of Gd was about seven except for a very small gadolinia content, suggesting that Gd ions are apt to form a Gd3+-vacancy-Gd3+ cluster. The reason why the diffusion constant at high gadolinia contents decreases was discussed in terms of short range and long range interactions. Comparison of diffusion behavior with yttria stabilized zirconia was also discussed.