Effect of particle size of raw powder on the sintering and grain growth of silicon nitride ceramics was invesitigated. β -silicon nitride powder, which is stable at high temperature, was used to eliminate the effect of phase transformation. Fine powder was made from commercial submicrometer powder by removing particles of over 0.5μm in diameter using grinding and centrifugal sedimentation. The fine and submicrometer powders were sintered by hot-pressing. Fine-grained silicon nitride ceramics was fabricated from the fine powder. This fine grained ceramics deformed at a strain rate of 1.8×10^-4/s under 1500°C and 40 MPa of compressive stress, i.e. it had superplasticity. Microstructure of the fine-grained ceramics was stable on annealing at 1800°C for lh. The grain growth rate during hot-pressing and annealing of submicrometer powder was larger than that in fine powder. It was revealed that the difference in grain size is a main driving force for the grain growth. Fine and homogeneous particle size distribution of raw powder is necessary to fabricate superplastic silicon nitride ceramics.