In order to clarify the influence of high temperature annealing on a next-generation gate insulating material YAlO3, single crystal YAlO3 samples were annealed at various temperatures from 900 to 1300°C in air for about 12 hours. The crystalline structure was examined by X-ray diffractometry, while the surface profile was examined by atomic force microscopy and a surface profilometer. Furthermore, infrared absorption spectroscopy and optical microscopy were used. As a result, the following serial changes in structure were estimated. The perovskite structure of crystalline YAlO3 starts to collapse and Y3Al5O 12 (YAG) is formed by the annealing at 1160 °C. Then, it segregates to Al2O3 andYAlO3 after the annealing at 1300 °C.