Mechanical properties of hybrid poly(methyl methacrylate) (PMMA) composites reinforced with poly- p-phenylene benzobisthiazole (PBO) fiber and nano-TiO2 (1, 3, and 5 vol.%) were investigated. In this work, the effect of PBO fiber surface treatment on tensile, flexural, and impact properties of PMMA composites were studied. The fiber loadings were varied from 0% to 20%. The addition of PBO fiber had caused a decline in the tensile strength of the PMMA composite. The three-point bending strength of the surface-treated PBO composites presents gradient increment along the TiO2 content, while the untreated one increases slowly. In case of treated PBO/PMMA composites, the impact strength noticeably increased with the PBO content. Results revealed that the presence of TiO2 on the surface-treated PBO fiber has further enhanced the efficiency of stress transfer from the matrix to the fiber thus improved the interfacial adhesion between the PBO fiber and PMMA matrix.