In this paper, single-phase Yb alpha-SiAlON ceramics were prepared by pressureless sintering and the densification, phase transformation as well as microstructure development were studied. It was demonstrated that the phase transformation could be completed at 1700 degrees C and elevated temperature was beneficial to grain growth. Additionally, effect of the addition of seed crystals prepared by combustion synthesis was discussed. From the experimental results, it was found that the added seed crystals facilitated the phase transformation and improved the microstructure development, resulting in the formation of elongated grains.