Alpha-SiAlON ceramics having various compositions and modifying cations were investigated with respect to their phase stability, transformation kinetics. and resulting microstructures. Each composition was heat treated at 150$0^{circ}C$ for 1h and measured the $alpha$-SiAlON transformation. The phase-boundary composition in the single-phase $alpha$-SiAlON region showed sluggish transformation from $alpha$-$Si_3N_4$ to $alpha$-SiAlON compared to the phase-center composition in the diagram. Using the different rare earth modifying cations, dependence of transformation kinetics on the phase stability in a fixed composition was also explained. By changing size of the stable u-phase region with exchanging cations, systematic change in transformation was observed. Transformation rate of $alpha$-SiAlON at low temperature has an important role on controlling the final microstructure. Less transformation gives more chances to develop elongated grain in the microstructure.