The wear behaviour of Ca a-sialon ceramics of two distinct microstructures, fine equiaxed grains (EQ) and large elongated grains (EL), with the same chemical composition was investigated as a function of apparent contact pressure and sliding speed, using ball-on-disc type tribometers at room temperature and at 600°C. For room temperature tests, the EL microstructure exhibited a lower wear rate than EQ in the severe wear regime due to a greater resistance to large crack-induced material removal. As the apparent contact pressure decreased, mild wear appeared for both microstructures. The mechanism that dominated the material removal in EQ was grain pullout. In contrast, the controlling mechanism for EL was transgranular fracture. Therefore, EL had a lower wear rate than EQ in the mild wear regime. For wear tests at 600°C, crack-induced severe wear occurs in both EQ and EL samples for all contact pressures. EL had a slightly lower wear rate than EQ. Wear particles were generated on the wear track, but no tribofilm was observed and no oxidation products were detected. Wear models revealed that the grain aspect ratio plays a more important role than grain diameter in influencing the crack propagation during severe wear and grain pull-out during mild wear.