Grain boundary microcracking in sliding test using a silicon pin was evaluated on several kinds of AlN to investigate the behavior of damage generation and the influence of material properties on it. The grain boundary fracture toughness (KIC gb ) was estimated from a percentage of the intergranular fracture as a grain boundary property for each specimen. KIC gb was greatly influensed by an amount and kinds of additives. After the sliding test, silicon debris accumulation and grain boundary microcracking of AlN were observed at the wear traces on the surface of AlN. The density of grain boundary microcracks increased linearly with increasing the contact load. Empirical relationship indicated that its density depended on KIC gb and the AlN grain size, and decreased mainly with increasing KIC gb. The density of grain boundary microcracks greatly degreased at the KIC gbvalue above 1.9 (MPa·m1/2).