In this study, the dependence of fracture stress on the crack length is examined at high temperature of 1100°C and 1200°C, and the availability of the process zone size fracture criterion for structural ceramics is studied. The mechanical properties of Si3 N4 at high temperature are compared with those of room temperature. It is shown that the process zone size fracture criterion is useful not only at room temperature, but also at high temperature. Furthermore, the effects of grinding depth and direction on the bending strength of ceramics are investigated. Various sizes of surface crack caused by the grinding process are estimated based on the critical process zone size fracture criterion by using the relation betweenσc and ae, and the Newman-Raju formula.