The effect of seeding on microstructure development and mechanical properties of silicon nitride was investigated by the use of two kinds of morphologically regulated β-Si₃N₄ single crystal particles. Silicon nitrides with bimodal grain-size distribution were obtained by seeding. The seeding of an appropriate amount of β-Si₃N₄ particles with an appropriate size allowed the development of optimal microstructures, where large elongated grains grown from the seed particles were highly dispersed. As a result, fracture toughness increased retaining the high strength levels of about 1GPa. On the other hand, specimens with large amount of seed particles presented lower strength owing to the coalescence of large elongated grains. The increase in fracture toughness can be associated with diameter as well as volume fraction of well dispersed large β-Si₃N₄ grains, assuming toughening by the bridging mechanism.