The mechanical properties of Si3N4 materials depend mainly on the microstructure, which originates during the densification process. The microscopic evidence indicates that beta -Si3N4 seeds incorporated in the starting powders play an important role in microstructural development, especially in the heterogeneous grain growth of beta -Si3N4 grains during sintering. The growth of beta -grains is initiated from the beta -seeds, resulting in a core/shell microstructure. The presence of Moire fringes and dislocations is attributed to misfit strain and compositional differences between the core and the shell. Coalescence can occur at the final stage of sintering.