Single-phase Yb alpha-SiAlON ceramics in-situ toughened by elongated grains were prepared by pressureless sintering, with the crystalline powder prepared by combustion synthesis used as seed crystals. The effects of seed addition on the densification, phase transformation, microstructure development, and mechanical properties of the samples were investigated in detail. From the experimental results, it was found that the added seed crystals could promote the phase transformation and facilitate the growth of elongated grains. By adding seed crystals the fracture toughness of the sintered alpha-SiAlON ceramics was considerably improved, with no obvious degradation in the bulk density and hardness at the same time. (c) 2006 Springer Science + Business Media, Inc.