Mesophase pitch-based large-diameter fibers were used to prepare unidirectional carbon/carbon (C/C) composite blocks, and mesophase pitch incorporated by various carbonaceous fillers was selected as a modified binder. The C/C composites were produced through a hot-pressing method, followed by carbonization and graphitization treatments. The influence of the carbonaceous fillers on the microstructure and physical properties of the C/C composites in different directions was investigated. The results show that both filler type and volume fraction have an important effect on the directional thermal conductivity of resulting composites. The through-thickness thermal conductivity of composites is significantly improved to be as high as 40-60 W/m K due to the incorporation of some carbonaceous dopants, which can tune their thermal conduction performance in different directions and thus solve the limitation of one-directional high thermal conduction. A three-dimensional carbon-based conductive network is diagramed to illustrate the thermal conduction mechanism of various carbon materials. The reinforcing effect on the directional thermal conductivity depends on the geometrical and physical characteristics, volume fractions and spatially distributed status of carbonaceous fillers. The flexural strengths of the composites are obviously improved owing to the decrease in porosity in the composites and the enhanced interfacial interaction between the carbon fibers and pitch-derived carbon matrix by doping with natural graphite flakes or carbon nanotubes.