Two types of commeriacl silicon nitride powders with poor dispersibilities have been studied, and influences of acid leaching and 950 ℃ thermal oxidation on the coprecipitation for the powders have been investigated sysmatically. The evolution of nano-oxides during the reaction causes the product to precipitation, and the calcinations of this precipitation gives nanocomposites powders(Al₂O₃-Y₂O₃) with extremely fine, uniform grains, and phase domains. microstructure properties of composites were investigated with scanning electron microscopy (SEM) (TEM) and Fourier Transform infrared spectroscopy (FTIR). The peak of the Si-O-Si groups emerge at higher frequencies after thermal oxidation, and the isoelectric point of 950 ℃ thermal oxidation powders was found to be less than 2, respectively. For the coprecipitation powders, the size of Al₂O₃-Y₂O₃ composites powders is 20~ 100nm.