The effect of $eta$-$Si_3N_4$ seeding on microstructural development of silicon nitride based materials has been investigated. In particular, to observe more distinctly the abnormal grain growth in pressureless sintering, fine $alpha$-$Si_3N_4$(mean particle size: 0.26 ${mu}m$) powder classified by sedimentation method was used. It was possible to prepare silicon nitride with abnormally grown grains under low nitrogen pressure of 1 atm thanks to the heterogeneous nucleation on $Si_3N_4$ seed particles. The size and morphology of silicon nitride grains were strongly influenced by the presence of $eta$-$Si_3N_4$ seed and overall chemical composition. For specimens with initially low $eta$-content, the large grains grew without a significant impingement by other large grains. On the contrary, for specimens with initially high $eta$-content, steric hindrance was effective. The resulting microstructure was less inhomogeneous and characterized by unimodal grain size distribution.