As hot pressing silicon nitride, sintering additives such as MgO, Al2O3 and Y2O3 are generally used. The use of sintering additives changes the physical properties of hot pressed silicon nitride markedly. In the present paper, silicon nitride was hot pressed with various amount of AlN as a sintering additive. Also mechanical properties such as density, hardness, Young's modulus, fracture toughness, flexual strength and thermal conductivity of hot pressed silicon nitrides were measured with regards to the content of AlN. Also microstructure such as grain-boundary phase was observed using a TEM. The effect of alpha/beta phase fraction on the mechanical and thermal properties of silicon nitride was investigated as a function of the amount of AlN additive.