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A proper powder preparation technique was used to develop elongated ␤-Si 3 N 4 particles as seeds from raw materials. The phase transformation and development of microstructure in Si 3 N 4 ceramics containing ␤-Si 3 N 4 seeds were investigated. The specimens of seeded silicon nitride had higher phase transformation rates than the specimens without seed. A core/shell structure was observed by transmission electron microscopy in seeded specimens owing to the difference in aluminum concentration in Si 3 N 4 grains. The misfit between the core and the shell was accommodated by interfacial dislocation that has a rotation character. The growth mode was epitaxial, although there was some compositional difference between the core and the shell. A relatively larger grain size and wider grain size distribution in seeded Si 3 N 4 specimens was due to the amount of ␤-phase seeds that could act as the nuclei, and the final modification of the microstructure was due to the coalescence process. The crack wake process characterized the mechanism of toughening.