Different microstructures in SIC ceramics containing Al2O3, Y2O3, and CaO as sintering additives were prepared by hot-pressing and subsequent annealing, The microstructures obtained were analyzed by image analysis. Crack deflection was frequently observed as the toughening mechanism in samples having elongated alpha -SiC grains with aspect ratio > 4, length > 2 mum, and grain thickness (t) < 3 mum (defined as key grains I), Crack bridging was the dominant toughening mechanism observed in samples having grains with thickness of 1 mum < t < 3 mum and length > 2 mum (key grains 2), The values of fracture toughness varied from 5.4 to 8.7 MPa.m(1/2) with respect to microstructural characteristics, characterized by mean grain thickness, mean aspect ratio, and total volume fraction of key grains. The difference in fracture toughness was mainly attributed to the amount of key grains participating in the toughening processes.