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Although significant increase in mechanical properties has been achieved in α-SiAlON ceramics by introducing self-toughening bimodal microstructure, the bimodal microstructure may not always result in better cutting performance of α-SiAlON tool inserts. Herein Dy-α-SiAlON ceramics were prepared by hot-pressing at 1900°C under 30MPa. Diameter and aspect ratio of the large elongated α-SiAlON grains pivotal to formation of bimodal microstructures were regulated by using 0-5wt.% Dy2O3 as a sintering additive. Mechanical properties and cutting performance of α-SiAlON tool inserts were evaluated. The results showed good Vickers hardness (19.0-19.7GPa) and fracture toughness (3.61-4.18MPa•m1/2) increasing with crack propagation length. However, transverse section of the large elongated grains was revealed as critical crack to cause premature fracture of the α-SiAlON ceramics. Fracture rather than attrition wear was recognized as dominant mechanism for tool failure, indicating that for better cutting performance of the α-SiAlON tool inserts a selftoughening microstructure consisting of thinner grains with large aspect ratio is required.