Densification behavior of $Si_3N_4$ ceramics by two step gas pressure sintering was compared with pres-sureless sintering one step gas pressure sintering or hot isostatic pressing. While it was difficult to get the highly interlocked ${eta}-Si_3N_4$ microstructure during the pressureless sintering due to decomposition above $1800^{circ}C$ gas pressure sintering could solve this problem by increasing the densification temperature 2MPa of nitrogen pressure was enough to inhibit the decomposition up to $1890^{circ}C$ and especially two step gas pres-sure sintering applying comparatively low pressure(2MPa) until the closed pore stage and then high pres-sure(10MPa) after pore closure could increase the hardness and the toughness.