Indentation tests on silicon nitride specimens using a silicon nitride sphere were carried out in order to establish a method for evaluating sphere indentation or impact damage of surface layers and post-indentation residual strength in structural ceramics. The residual strengths of indented specimens were measured by four-point bending tests. The relationship between indentation load and cone crack length was derived from the equilibrium condition of cone crack. Then the residual strength was estlmated from the cone crack length. The estimated values were in good agreement with the was estimated from the cone crack length. The estimated values were in good agreement with the measured values. only in the region of low indentation load where elastic deformation is dominant.