In the present study, the phase composition and generation mechanism of the nanocrystals located in the microcracks of plasma-sprayed Al2O3 coating were reevaluated. The Al2O3 coatings were investigated using transmission electron microscopy and x-ray diffraction. We supply the detailed explanations to support the new viewpoint that in situ formation of alpha-Al2O3 nanocrystals in the preexisting microcracks of the as-sprayed Al2O3 coating may be due to the stress-induced phase transformation. Owing to the partially coherent relationship, the phase interfaces between the alpha-Al2O3 nanocrystals with the preferred orientation and the c-Al2O3 matrix may possess better bonding strength. The alpha-Al2O3 nanocrystals could repair the microcracks in the coating, which further strengthens grain boundaries. Grain boundary strengthening is beneficial to the coating fracture toughness enhancement.