Boron-doped carbon-carbon composites with boron concentration around 11-15 mass % were prepared from a carbon fibre felt with dispersed boron carbide powder by infiltration of pyrolytic carbon. The composite was heat treated at several different temperatures from 2000-2800 degrees C. The highest bending strength was obtained for the composite at a heat-treatment temperature (HTT) of 2200 degrees C. Carbon fibre began to be destroyed after heat treatment at 2400 degrees C and the structure of the composite was drastically changed above 2600 degrees C where the anisotropy of the composite originally existing in the thermal expansion coefficient and the thermal conductivity has been faded away. X-ray diffraction measurement indicated that graphitization of the composite was enhanced by boron doping. At HTTs above 2400 degrees C, the composite became graphitic, the crystallite sizes of which were more than 100 nm in L(c) (004) and L(a) (110). It was shown that boron was uniformly distributed in the composite at an HTT of 2400 degrees C and also that heat treatment at higher temperatures, such as 2600 degrees C, incurred condensation of boron. Air-oxidation loss at 800 degrees C appeared to be the lowest for the composite with an HTT of 2400 degrees C and the rate of oxidation loss was 22 times lower than that of the non-boron-doped composite.