SiAlON-based ceramics are some of the most typical oxynitride ceramic materials, which can be used as cutting tools for heat-resistant super-alloys (HRSA). SiAlON can be fabricated by using gas-pressure reactive sintering from the raw materials, nitrides and oxides such as Si3N4, AlN, Al2O3, and Yb2O3. In this study, we fabricate Ybm/3Si12-(m+n)Alm+nOnN16-n (m=0.3, n=1.9, 2.3, 2.7) ceramics by using gas-pressure sintering at different sintering temperatures. Then, the densification behavior, phase formation, microstructure, and hardness of the sintered specimens are characterized. We obtain a fully densified specimen with β- SiAlON after gas-pressure sintering at 1820℃ for 90 min. under 10 atm N2 pressure. These SiAlON ceramic materials exhibited hardness values of ~92.9 HRA. The potential of these SiAlON ceramics for cutting tool application is also discussed.