Postirradiation annealing tests were performed to obtain the 133Xe diffusion coefficients in uranium dioxide (UO2) and mixed thorium-uranium dioxide [(Th-U)O2] fuels. Specimens were a single-grained UO2, a polycrystalline UO2, and a polycrystalline (Th-U)O2. The (Th-U)O2 specimen was a mixture of 35% ThO2 and 65% UO2. Each 300-mg specimen was irradiated to a burnup of 0.1 MWd/t U. Postirradiation annealing tests were performed at 1400, 1500, and 1600°C, continuously. The xenon diffusion coefficients for the nearly stoichiometric single-grained UO2 agree well with the data of others. The xenon diffusion coefficients in the polycrystalline (Th-U)O2 are approximately one order lower than those in the polycrystalline UO2. The xenon diffusion coefficient in the (Th-U)O2 increases with the increasing oxygen potential of the ambient gas.