Applications of semiconductor power modules are rapidly expanding in a broad range of fields such as power generation systems and electric transportation systems. With increasing power supply and packing density of power devices, the ceramic boards are required to have high strength and high toughness as well as high thermal conductivity. Silicon nitride is an attractive candidate material because of its excellent mechanical properties combined with high intrinsic thermal conductivity over 200 Wm^-1K^-1. However, thermal conductivities of silicon nitride materials fabricated via the conventional sintering method are insufficient. This is because impurity oxygen dissolved in Si₃N₄ grains causes phonon scattering to lower their thermal conductivities. In order to decrease dissolved oxygen, high purity silicon powder was employed as a starting raw material. We have succeeded in preparing Si₃N₄ with a very high thermal conductivity of 177 Wm^-1K^-1 and good mechanical properties by nitriding a silicon powder compact followed by post-sintering.