The Si5AION7 specimens containing 20% YAG composition have been sintered and then heat treated to induce the crystallization of the matrix glassy phase. Crystallization of YAG phase during the heat treatment was detected from the X-ray diffraction patterns and the consequent changes in room temperature toughness as well as in microstructures were investigated. Almost all the glassy boundary phase were found to crystallize even after 5 minutes of heat treatment and the KC at room temperature decreased accordingly. The results show that the matrix crystallization can be induced by slow cooling from the sintering temperature.