A microscopic theory of thermally induced crack healing in poly(methyl methacrylate) is presented. Both laser-induced cylindrical cracks and knife-induced surface cracks were analyzed. For a given temperature, the crack closure rate was constant for both types of cracks. However, the crack closure rate was lower for samples with cylindrical cracks than for those with surface cracks. The former exhibited higher activation energy for crack closure than the latter, because the knife-induced cracks had sharper crack tips. Fracture stress was proportional to surface crack healing time to the one-fourth power for thermal healing at a given temperature. Based on the reptation model of polymer chains, the activation energy of chain diffusion was calculated. The healing process was monitored via fractography and crack closure was confirmed. The results were compared with solvent healing and thermal healing in the literature.