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Silicon nitride is a typical engineering ceramics as structural materials for high-temperature or functional applications. However, the fabrication of silicon nitride costs a lot because the densification of silicon nitride needs vast energy to get high-temperature of 2073~2273 K and special apparatus such as a hot-press. Therefore, it is difficult to use silicon nitride in large amount. Consequently, lowering of the fabrication cost is essential to apply silicon nitride to wider fields.We propose a new process of the liquid phase sintering of silicon nitride using an oxynitride sintering aid which consisted of SiO 2 , MgO and preliminarily added α-Si 3 N 4 and was fired at temperature up to 2073 K. The α-Si 3 N 4 green compacts with the oxynitride sintering aid were sintered at 1873 K for 64-512 min under 0.1 MPa nitrogen pressure, and fully dense β-Si 3 N 4 ceramic was fabricated after 512 min sintering. This sintering temperature is about 200 K lower than that of a conventional method.