Abstract: Three-dimensional (3D) carbon/carbon (C/C) composites with high thermal conductivity were prepared from a preform prepared by orthogonally weaving continuous mesophase pitch-based fibers in the x and y directions and commercial PAN-based carbon fibers in the z-direction, which were densified by three cycles of chemical vapor infiltration (CVI) and graphitization to a density of 1.58 g/cm³ (3CVI), followed by four cycles of liquid pressure impregnation (LPI), carbonization and graphitization to give a density of 1.84 g/cm³ (3CVI+4LPI). The effects of the microstructure and the relative contributions of the fibers and matrix carbon to the thermal conductivity and mechanical properties of the C/C composites were investigated. Results indicate that the CVI pyrolytic carbon (PyC) is highly crystalline and oriented along the fiber axis. The thermal conductivities of the 3CVI and 3CVI+4LPI C/C composites in the x-y plane are respectively 115.9 and 234.7 W/m·K, while those in the z-direction are only 18.6 and 41.5 W/m·K. The thermal diffusivity and thermal conductivity mainly depend on the fiber type, the volume fractions of the fibers and the type of pyrolytic carbon. The thermal conductivity of the composites is improved by increasing the volume fraction of pitch-based carbon fibers and using matrix carbon that is easily graphitized. The mechanical properties of the two C/C composites are greatly improved compared with those of 1D-C/C and 2D-C/C composites.