The effect of lanthanum (La) salt on surface treatment of poly-p-phenylenebenzobisoxazole (PBO) fiber on flexural and tribological properties of PBO fibers reinforced polyimide (PI) composites was investigated. Experimental results revealed that flexural strength and modulus were largely improved by La salt treatment. The La salt treated composite had lower friction coefficient and specific wear rate than that of untreated one under the same testing condition. The principle of improvement in interfacial adhesion between PBO fiber and PI matrix after La salt treatment was discussed. The surface characteristics of PBO fibers were characterized by X-ray photoelectron spectroscopy. It was found that the content of polar groups on the surface of PBO fiber treated by La salt increased compared with the untreated fiber. The increase in the amount of polar groups strengthened the interfacial adhesion between PBO fibers and PI matrix, and accordingly enhanced the flexural and tribological properties.