In order to clarify mechanisms of surface damage and to establish methods for evaluating strength degradation in ceramics subjected to impact of solid particles, testings of silicon nitride sphere impact to silicon nitride specimens were carried out. The residual strengths of specimens were measured by flexural tests, in which the damaged surfaces were the tensile sides. Fractures were classified to several types depending on their own impact damages. Influences of specimen thickness on the impact damage and the residual strength were also examined. From the comparison of test results of indentation and impact, the possibility was shown that the residual strength due to sphere impact would be evaluated using a method based on the elastic indentation theory.