Sintered composites in the B6O-xdiamond (x = 0-80 vol%) system were prepared under high pressure and high temperature conditions (3-5 GPa, 1400-1800 degreesC) from the mixture of in-laboratory synthesized B6O powder and commercially available diamond powder with various grain sizes (<0.25, 0.5-3, and 5-10 mum). Relationship among the formed phases, microstructures, and mechanical properties of the sintered composites was investigated as a function of sintering conditions, added diamond content, and grain size of diamond. Sintered composites were obtained as the B6O-diamond mixed phases when using diamond with grain sizes greater than 0.5 mum, while the partial formation of the diamond-like carbon was observed when using diamond grain sizes less than 0.25 mum. Microhardness of the sintered composite was found to increase with treatment temperature and pressure, and the fracture toughness slightly decreased. A maximum microhardness of H-v similar to 57 GPa was measured in the B6O-60 vol% diamond (grain size < 0.25 mum) sintered composite under the sintering conditions of 5 GPa, 1700 degreesC and 20 min.