B6O powders were hot pressed with and without Al2O3 as a sintering additive at temperatures up to 1900 degrees C and a pressure of 50 MPa. The microstructure of a doped and undoped sample was studied by transmission electron microscopy techniques. This paper aims at studying the correlation between micro/nanostructure evolution and the resulting mechanical properties; i.e., hardness and fracture toughness. The addition of alumina yields the formation of a secondary aluminum borate phase in addition to promoting grain growth strongly. While the addition of Al2O3 slightly decreased the hardness of the B6O polycrystals, the corresponding fracture toughness was strongly improved, as compared with the undoped material.