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A detailed study has been undertaken on crack healing at high temperatures in a silicon nitride containing 10 wt% additives in order to identify the dominant mechanism responsible for the phenomenon. Fracture toughness increased with annealing time and the crack growth rate decreased until arrest with increasing testing time. Differentiation between possible operating mechanisms was obtained using critical experiments involving detailed compliance measurements, crack wake removal, and crack reinitiation tests and a comprehensive TEM study of healed cracks. It was found that crack healing was not uniform in the crack wake. When the original crack path was either transgranular or intergranular, healing was associated with the appearance of a thin layer of silica glass due to the oxidation of Si 3 N 4 grains. But when the crack went through multigrain junctions, the former crack path was completely obliterated and replaced by a new, crystalline phase formed by diffusion of the preexisting glass phase. It is concluded that the increased crack growth resistance and fracture toughness at high temperature is attributable to the partial recovery of the original strength from the crack segments at multigrain junctions due to vitreous phase flow and subsequent crystallization.