The chemical diffusion coefficients of oxygen were measured for the (Th1-y, U-y)O2+x with uranium content y of 1, 3 and 5 mol% in the temperature range of 1213-1313 K under the oxidizing or reducing atmosphere. The magnitude of activation energies obtained for oxygen diffusion was in good agreement with the migration energy which is calculated based on interstitialcy mechanism by Colbourn. With increasing uranium content, the chemical diffusion coefficient increased and the activation energy for oxygen diffusion decreased slightly in the present experimental conditions. The excess oxygen were soluble into the (Th1-y, U-y)O2+x sample, so that the interaction energy between the higher valence uranium ions and interstitial oxygen ions might increase with decreasing uranium content. The effect of oxygen partial pressure on both the chemical diffusion coefficient and the lattice parameter of (Th1-y, U-y)O2+x could not be found. The lattice constant decreased linearly with uranium content, which indicated that the relation between these parameters obeyed Vegard's law.