Three types of β-SiAlON (Si6-z AlzOzN8-z) powder were prepared by the carbothermal reduction and simultaneous nitridation of ultrafine powders in the Al2O3-SiO2 system. The ultrafine starting oxide powders, prepared using the vapour-phase reaction technique, were mixed with carbon powder and heated at 1400 °C for 1 h under flowing nitrogen to form β-SiAlON and followed by heating at 570 °C for 1 h in air to remove residual carbon. The resulting powders contained only β-SiAlON with z values of 1.63, 2.05, and 2.99. The relative density (bulk density/true density) of β-SiAlON compacts pressureless sintered at 1800 °C for 1 h under flowing nitrogen increased with z and reached 89.9% at z = 2.99. When the β-SiAlON compact with z = 2.99 was hot pressed at 1800 °C for 1 h under flowing nitrogen, a maximum relative density of 93.6% was achieved. Although this hot pressed compact contained a small amount of 15R-SiAlON in addition to β-SiAlON, it possessed a small average grain size (typically 0.5 μm diameter) and high Vickers hardness (19.2 GPa).