Sintered composites in the B6O-xB(4)C (x = 0-40 vol%) system were prepared under high pressure and high temperature conditions (3-5 GPa, 1500-1800 degrees C) from the mixture of in-laboratory synthesized B6O powder and commercially available B4C powder. Relationship among the formed phases, microstructures and mechanical properties of the sintered composites was investigated as a function of sintering conditions and added B4C content. Microhardness of the sintered composite was found to increase with treatment temperature up to 1800 degrees C, while fracture toughness decreased slightly. Maximum microhardness of Hv similar to 46 GPa was obtained from B6O-30vol%B4C sintered composite under the sintering conditions of 4 GPa, 1700 degrees C and 20 min.