The effect of Y₂O₃/SiO₂ ratio on grain growth of Si₃N₄ was systematically investigated using the “seed” fabrication method. The aspect ratio of β-Si₃N₄ crystals heat-treated with an additive composition of Y₂O₃:SiO₂=1:2 was found to be much smaller than that of Y₂O₃:SiO₂=2:1. It was confirmed that this was due to the difference in solubility of Si₃N₄ caused by the change of Y₂O₃/SiO₂ ratio of the liquid phase. The number of β-crystals originally contained in the raw Si₃N₄ powder changed very little in the Y₂O₃:SiO₂=1:2 composition, while it significantly decreased in the Y₂O₃:SiO₂=2:1 composition, which resulted in a considerable difference in aspect ratio. A mechanism for this enhanced solubility of Si₃N₄ in the liquid phase with high Y₂O₃/SiO₂ ratio was proposed.