Seeding effects on the microstructure and mechanical properties of single-phase Ca-alpha-SiAlON ceramics have been investigated. Whereas a small amount of seeds can transform the microstructure from one of fine equiaxed grains to one consisting of many needle-like grains, the highest fracture toughness of 8 MPa.m(1/2) is not reached until 8% seeding. This contrasts with the much higher seed efficiency in Y-SiAlON, where the peak toughness is reached at 1% seeding. The difference and the general trend of mechanical properties of seeded alpha-SiAlONs are discussed in terms of alpha-SiAlON formation and toughening mechanisms.