The mechanical strength of carbon fiber reinforced thermoplastic (CFRTP) composite material is influenced by the interfacial adhesion strength of carbon fiber (CF) and matrix resin. The sizing agent coating a CF surface affects the interfacial characteristics between the CF and matrix resin. That is, the sizing agent affects the mechanical strength of CFRTP. Therefore, it is necessary to treat the sizing-agent-coated CF prior to CFRTP molding. In this study, we investigated the effect of heat treatment of sizing-agent-coated CF on the flexural strength of CFRTP and the interfacial shear strength between CF and thermoplastic matrix resin. As a result, it was found that the heat-treatment conditions of CF that maximize the flexural strength of CFRTP and the interfacial shear strength of CF and thermoplastic resin depend on the composition of the thermoplastic resin. Furthermore, a correlation was found between the flexural strength of CFRTP and the interfacial shear strength between CF and matrix resin. That is, by applying heat treatment suitable for the composition of the thermoplastic resin to the CF, the interfacial shear strength was increased and the flexural strength of the CFRTP was increased.