Silicon nitride containing 0.5-10mol% of Y₂O₃-Nd₂O₃ was fired at 1600°-1900°C in 10MPa N₂ to investigate the lowest limit of additives to densify Si₃N₄ using gas pressure sintering. One mol% oxide additive was found to be sufficient for densification. This material fired at 1900°C for 4h was densified to 97% of theoretical with the average bending strength of 784MPa.