Abstract High-temperature creep properties of sintered uranium dioxide pellets with two grain sizes (9.0 μm and 23.8 μm) were studied. The results indicate that the creep rate becomes a little faster with the reduction of the uranium dioxide grain size at the same temperature and the same load. At the same temperature, the logarithmic value of the steady creep rate vs stress has linear relation, and with increasing load, the steady creep rate of the sintered uranium dioxide pellet increases. Under the same load, the steady creep rate of the sintered uranium dioxide pellet increases with increasing temperature; and the creep rates of sintered uranium dioxide pellet with the grain size of 9.0 μm and 23.8 μm under 10 MPa are almost the same. The creep process is controlled both by Nabarro—Herring creep and Hamper—Dorn creep for uranium dioxide pellet with grain size of 9.0 μm, while Hamper—Dorn creep is the dominant mechanism for uranium dioxide with grain size of 23.8 μm.