Recovery of mechanical strength in ¡-Al 2 O 3 -based hybrid materials with 5 vol% dispersed nano-Co particles was carried out via a thermal oxidation process. Three Vickers indentations were conducted at 49 N for 10 s in air to introduce pre-cracks on the sample surface. The bending strengths of as-sintered and as-cracked samples were approximately 710 and 159 MPa, respectively. Bending strength value increased to 636 MPa when as-cracked specimens with introduced surface cracks were heat-treated at 1200°C for 6 h in air. Surface cracks disappeared completely through formation of the oxidation product, CoAl 2 O 4 . Specimens heat-treated at 1200°C for 6 h in air did not fracture through the surface cracks introduced by the Vickers indentation during three-point bending test. Specimens heat-treated at 1200°C for 6 h in an Ar10%H 2 gas flow, in which metallic Co is stable, were broken along the surface crack introduced by the Vickers indentation. The recovery of mechanical strength was obtained by surface cracks filling with CoAl 2 O 4 .