The effects of the starting particle size and / phase transformation during sintering process on the microstructure evolution of Yttrium -Sialon ceramics were investigated. As-received powder (mean particle size: 0.54m) and classified powder(mean particle size: ) were used as starting powders. With decreasing the starting particle size, the growth of elongated grains was enhanced, which resulted in the whisker -like microstructure with elongated grains. These results were discussed in relation to the two-dimensional nucleation and growth theory. In the specimen heat treated at for 10h before sintering at for 1h under 40atm(2-step sintering), the grain size was smaller than of the 1-step sintering at 195 for 1h. However, bimodal microstructure evolution were not not remarkable in both sample, which is ascribed to the -phase contents existing in starting powder.