Mechanical and tribological properties of polymethyl methacrylate (PMMA) composites filled with titanium oxide (TiO₂) were studied with particular interest on the HCl surface treatment. Increasing the TiO₂ content in the PMMA matrix from 1 wt% to 7 wt% resulted in improved tensile strength and then decreased. The tensile and tribological properties show the optimum TiO₂ content is obtained at 3 wt%. HCl treatment largely improved the friction and wear of TiO₂/PMMA composites. Scanning electron microscope (SEM) investigation of worn surfaces of PMMA composites showed that HCl-treated TiO₂/PMMA composite had strong interfacial adhesion and smooth worn surface under given load.