A sintered silicon nitride excellent in high temperature performance is produced by the following method: First, silicon nitride powder and oxide of at least one element selected from elements in the group IIIb of a periodic table are mixed with each other to obtain a mixture. The silicon nitride powder contains silicon oxide in an amount ranging from 0.1% by weight of the silicon nitride powder to a value not more than a content of the silicon oxide. Second, the mixture is compacted to form a compact. Finally, the compact is fired in atmosphere of nitrogen at a pressure ranging from 5 to 200 atmosphere (atm) and a temperature ranging from 1800.degree. to 2000.degree. C. to obtain a sintered silicon nitrite having a bulk density not less than 95% of a theoretical density of the sintered silicon nitride.