Bending strength, fracture toughness, fracture energy and crack extension resistance were evaluated for Al2O3 ceramics with equi-axed and platelet grains. Bending strength was proportional to grain size−1/2, but flaws with a size of 10 μm controlled the strength when the microstructure was finer than 10 μm. Fracture toughness, measured by the single etched precracked beam (SERB) method, was proportional to fracture energy1/2, and increased with the grain size of Al2O3 ceramics with equi-axed and platelet grains. However, the toughness of platelet grain ceramics was higher than the ceramics with equi-axed grains, and increased up to 6.6 MPam1/2 with grain size. Therefore, it is thought that fracture toughness not only depends on grain size, but also on grain morphology; equations were derived to account for this phenomenon.