High-temperature tensile ductility of Al₂O₃ is much enhanced by spinel (MgO·1.5Al₂O₃) dispersion. However, the enhanced tensile ductility in Al₂O₃-20vol% spinel cannot be explained solely from grain size stability or reduction of flow stress. Detailed microstructural analysis reveals that the crack-like cavity growth rate during high-temperature deformation is more sluggish in Al₂O₃-20vol% spinel than in 0.1wt% MgO-doped single-phase Al₂O₃. The enhanced tensile ductility in Al₂O₃-20vol% spinel must be explained from the reduction of crack-like cavity growth rate during deformation. Since the grain boundaries in Al₂O₃-20vol% spinel mainly consist of Al₂O₃ grain boundaries and Al₂O₃/spinel interphase boundaries, the origin of the sluggish crack-like cavity growth in Al₂O₃-20vol% spinel is due to high resistivity against crack growth in Al₂O₃/spinel boundaries. Detailed TEM analysis clarifies that there is an epitaxial relationship between Al₂O₃ and spinel grains in many Al₂O₃/spinel interphase boundaries. These interphase boundaries are expected to have smaller interfacial energy, which must act to retard the crack-like cavity growth in spinel dispersed Al₂O₃.