Silicon nitride with various amount of AlN as a sintering aid was sintered by a hot press method. Densified silicon nitrides were obtained, and it was found that the mechanical and wear properties were dependent on the contents of AlN. The effect of a/b phase on the mechanical and wear properties of silicon nitride was investigated. The properties were changed depending on the amount of a/b phase. In the brittle materials, tribological behaviors were dependent on the microstructure as well as hardness and fracture toughness. We focus on the relationship between the microstructure and mechanical/wear properties of silicon nitride including AlN additives.