B(4)C matrix composites containing 0.5-10 vol% carbon nanotubes (CNTs) were fabricated by hot-pressing using Al additive and the effects of CNT addition on mechanical and thermal properties of the B(4)C/CNT composites were investigated for improvement of control material of fission reactors. Bulk densities of the composites decreased with increasing the amount of CNT. The CNTs in the composites were orientated slightly perpendicular to the hot-pressing direction and the anisotropic microstructure was formed. Vickers hardness and elastic modulus of the composites decreased with an increase in the amount of CNT. Fracture toughness of the composites was slightly improved by CNT addition, and at high CNT contents, bridging by CNT was observed in the cracks. Thermal conductivities of the composites increased slightly as compared with those of B(4)C without CNT. However, clear improvement effect by the CNT addition on the thermal conductivities of the composites was not observed within the evaluated compositions.