In carbon fiber reinforced thermoplastics (CFRTP) employing polyamide 6 (PA6) as the matrix resin, we found that the reduction in CFRTP fiber length due to mechanical recycling not only decreased the impact strength but also accelerated the thermal oxidation of the matrix resin. In a heating test of an injection molded CFRTP specimen prepared by crashing and remolding, a portion of PA6 was insoluble in the solvent and in a state in which carbon fibers were entrapped. With the results of a heating test employing model samples with different fiber length, we also confirmed that thermal oxidation was accelerated as the fiber length was reduced. There was no significant correlation between the length of the carbon fibers and the number of radicals; but the shorter the fiber length, the greater the number of acidic functional groups on the surface of the carbon fiber. Therefore, we inferred that the insolubilization was mainly caused by the reaction between functional groups in the carbon fiber and PA6.