Abstract: The UO₂-MWNTs (multi-walled carbon nanotubes) composite fuel was designed and prepared, the thermal conductivity of the composite fuel was studied, and the key factors affecting the thermal conductivity of the composite fuel were analyzed. The results show that the thermal conductivity of composite fuel is mainly related to the volume content of MWNTs, the interface and the length-to-diameter ratio of MWNTs. The thermal conductivity of composite fuel increases with MWNTs volume content. When the volume content is more than 10%, the thermal conductivity of the composite fuel is further enhanced; The phase interface of UO₂ and MWNTs have a significant effect to the thermal conductivity. At above 1 521 ℃, UO₂ and MWNTs react to produce UC which hinder the improvement of the thermal conductivity. At 1 450 ℃, the thermal conductivity of hot pressed sintered pellet increases by 25.65%, and at 1 650, 1 700 and 1 750 ℃, the thermal conductivities of pressureless sintered pellets increase by 22.67%, 20.32% and 18.3%. The thermal conductivity of the composite fuel increases with length-to-diameter ratio. When the length-to-diameter ratios of MWNTs are 3×10³, 5×10³, 7×10³ and 9×10³, the thermal conductivities increase by 18.3%, 19.93%, 22.42% and 25.09% respectively.