Techniques to fabricate thorium-uranium dioxide fuel [(Th,U)O2] have been developed, and the thermal conductivity of (Th,U)O2 pellets has been measured. Mixtures of thorium dioxide (ThO2) and uranium dioxide (UO2) powders were successfully wet-milled, compacted, and sintered at 1700°C to fabricate (Th,U)O2 pellets. The wet-milling process results in a fuel density of 96 to 98% of theoretical density and a uniform distribution of the uranium and thorium in the (Th,U)O2 pellet. The laser flash method was used to measure the thermal diffusivity of the ThO2 and (Th,U)O2 pellets, and the thermal conductivities of (Th0.655U0.345)O2 and (Th0.355U0.645)O2 fuel were found to be lower than that of ThO2 or UO2 fuel. The degradation of the thermal conductivity by the UO2 substitution is great at low temperatures but decreases as the temperature increases.