The surface functionalization of titanium dioxide (TiO₂) was used to modify the surface of carbon fiber (CF). The objective of this study is to improve the interlaminar shear strength and impact properties of the composites by mixing high-density polyethylene (HDPE) resin and modifying CFs. The Izod impact strength of the CF/HDPE composites increases with increasing CF content because of the high impact strength of the CF, whereas incorporation of the TiO₂ increased the impact strength of the CF/HDPE composites, which may be attributed to the reinforcing effect of the TiO₂ particles. Especially the surface treatment of TiO₂ increases the interfacial adhesion and the thermal stability of the CF/TiO₂/HDPE composite.