B6O sintered compacts were prepared by high pressure and temperature treatments of in-laboratory synthesized B6O powder. Sinterable B6O powder with grain sizes of 1.2-1.4μm was formed above 1300°C in 4 hours in a pure argon stream by a solid-liquid reaction between amorphous boron and B2O3 powders. Single phase sintered compacts with relative densities above 95% were obtained under the sintering condition of 3-5GPa, 1700°C and 20min. The Vickers microhardness of the sintered compact was 31-33GPa. The B6O sintered compact exhibited an oxidation resistance in air up to 600°C and a mild oxidation behavior in the temperature range of 700-1000°C with the formation of a liquid protective film of B2O3.