The erosion rates and impact damage of two sintered Si3N4 materials with identical compositions but different microstructures were determined using a gas-blast-type erosion rig. The erodent particles used were SiC grits and the impact angles investigated were 30degrees and 90degrees. It was found that the erosion behavior of the two materials could not be related to their mechanical properties, such as hardness and fracture toughness as predicted by the theoretical erosion models. In fact, a close relationship was identified between their microstructure and the erosion mechanism. Microstructures containing evenly dispersed and uniaxially oriented reinforcing whiskers promoted grain-pullout, while the randomly oriented elongated grains hindered it.