High-pressure sintering behavior in the B6O-c-BN system was investigated using in-laboratory-synthesized B6O and commercially available c-BN powders (with an average grain size of 0.5, 3, or 6 mu m) No reaction occurred between the two components under the high-pressure (4-6 GPa) and high-temperature (1500 degrees-1800 degrees C) conditions that have been investigated. Well-dispersed, sintered B6O-x(c-BN) composites (where x = 0-60 vol%) of almost-fuil density were prepared by sintering at a pressure of 6 GPa and temperature of 1800 degrees C for 20 min, The maximum Vickers microhardness (46 GPa) of these composites was attained by adding 40 vol% c-BN with an average grain size of 0.5 mu m. The fracture toughness of these composites increased as the c-BN content increased; the maximum fracture toughness (1.5-1.8 MPa.m(1/2)) was observed for x = 40-60 vol%. Crack deflection along the B6O-c-BN grain boundary contributed to increasing the fracture toughness.