Sintered silicon nitride, particularly in structural ceramics, has superior properties such as low weight, heat resistance, wear resistance, etc. It is already being applied to automobile engine parts such as the swirl chamber and the turbine rotor.In recent years, the strength of silicon nitride has shown to be above 1000MPa. This has been achieved through advances in manufacturing technology such as materials powder, forming, sintering and so on. But the silicon nitride is easily damaged during grinding because it has less fracture toughness than metal. Consequently, the inherent strength of the material is not demonstrated in the actual products presently produced. It is assumed that the main cause of strength reduction is microcrack. In ordinary grinding methods, the length of microcrack has been estimated at approximately twenty micrometers by fracture mechanics analysis. But grinding damage has not yet been satisfactorily determined because microcrack evaluation methods have not been established (1)(2).Analysis of grinding damage is of great importance in developing highly reliable ceramic parts. We have developed a new metallic solution impregnation method which can be used as a microcrack evaluation method. By this method, the influence of crack depth and residual stress distribution on the strength of the grinding surface can be analyzed.