Five kinds of silicon nitride samples with different microstructure were prepared in order to examine the effect of sintering condition on grain size and mechanical properties. The degradation in strength of ceramics caused by grinding is investigated in detail. It is shown that the critical process zone size is closely related to the grinding process. Futhermore, the crack size caused by grinding is estimated by the process zone size failure criterion and the Newman-Raju formula. It can be concluded that the process zone size failure criterion is useful when evaluating the sensitivity of ceramics to grinding.