Si 3 N 4 ceramics with a constant molar fraction of Al 2 O 3 and Y 2 O 3 or Yb 2 O 3 have been microstructurally characterised by scanning electron microscopy in combination with quantitative microscopy, and the three-dimensional β-Si 3 N 4 grain size distributions were subsequently reconstructed by a stereological method. The Si 3 N 4 ceramics were densified by gas pressure sintering at 1900°C under a pressure of 980 kPa for 1, 3, 5 and 10 h. The microstructural analysis showed that the Y 2 O 3 Al 2 O 3 ratio in the starting powder mixture affects the β-Si 3 N 4 grain shape and size distribution. An increased Y 2 O 3 Al 2 O 3 ratio resulted in a higher mean aspect ratio. A replacement of Y 2 O 3 by Yb 2 O 3 increased the mean aspect ratio further and resulted in a more narrow grain size distribution with an increased mean grain volume. The mean grain size increased with densification time, and a grain growth exponent of n = 3 2 implied that the β-Si 3 N 4 grain growth was rate controlled by diffusion through the liquid phase. The metal oxide composition also determined the formation of secondary crystalline phases. Crack propagation and, hence, fracture toughness was dependent upon β-Si 3 N 4 grain morphology and the intergranular structure.