The search for suitable additives for boron suboxide (B6O) materials which could improve densification, reduce sintering temperature and tailor the microstructure has been of great importance. B6O materials doped with 0-5 vol% cobalt addition were sintered at temperatures up to 1850oC and pressure of 50MPa for 20mins. Relationship among the formed phases, microstructures and mechanical properties of the sintered materials was investigated as a function of sintering conditions and added cobalt content. The hardness of the sintered B6O materials increases with sintering temperature, while it decreases with increase in cobalt content. The reverse is also true for the fracture toughness.