The phase stability of α-sialon has been investigated on a Y-α-sialon-Y 2O 3Al 2O 3 plane. Y-α-sialon was stable with YAG liquid composition at 1750 °C which could be devitrified to YAG (3Y 2O 3·5Al 2O 3) by post heat treatment at 1500 °C for 4 h. With increase of Al 2O 3 in the liquid composition of YAG, the amount of β-sialon increased. On the other hand, with increase of Y 2O 3 in the liquid composition, the amount of melilite (Si 3N 4 · Y 2O 3) increased. The changes of unit-cell parameters and microstructures with respect to the compositions and amounts of the liquid phase were also discussed.