For two different alpha-Sialon ceramics, one with a stoichiometric composition and the other one with an extra amount of yttria, the fracture toughness (SEVNB) and the crack opening displacements (COD) in the crack tip (CT) region were measured. The fracture toughness values were 3.0 MPa root m for the material without excess Y2O3 and 4.5 MPa root m for the material with a residual grain boundary phase. In contrast, the crack tip toughness of alpha-Sialon with the stoichiometric composition was 1.35-1.68 MPa root m and 0.95-1.01 MPa root m for the alpha-Sialon with an excess amount of yttria. Crack behavior in alpha-Sialon with an excess amount of yttria is both intergranular and transgranular owing to the formation of large elongated grains, and mainly intergranular in alpha-Sialon with stoichiometric composition. The different crack behavior and the resulting crack tip toughness are the result of the different chemistry and changed microstructure of the materials.