The microstructural development of Ca α-sialon ceramics has been studied. It was found that the microstructure of these materials could be controlled by the starting compositions and the processing conditions. Consequently, the observed microstructure has been related to the mechanical properties of these materials. It was found that although fracture toughness increased with increasing grain size and aspect ratio, a weakened interface between the α-sialon grains and intergranular glass is essential to further improve the toughness of these materials.