A composite of poly p-phenylene-2,6-benzobisoxazole (PBO) fiber and epoxy resin has excellent electrical insulation properties. However, it is a challenging issue to improve its mechanical properties because of poor adhesion between PBO fiber and matrix. The relatively smooth and chemically inactive surface of PBO fiber prevent efficient chemical bonding in the composite interface. Here, we report the surface modification of PBO fibers by UV irradiation, O-2 and NH3 plasma, as well as acidic treatments. We found that the surface free energy and roughness are increased for both sized and extracted fibers after plasma treatments together with maleic anhydride grafting. The sized fiber shows marginal improvement in adhesion strength and no change in fiber tensile strength because of the barrier effect of the finish. For the extracted fiber, however, the tensile strength of the fiber is sensitive to surface treatment conditions and considerable strength reduction occurred, particularly for cases of acidic treatments and UV irradiation. This is because that the treatments increase the surface roughness and introduce more surface flaws. The extracted fiber surface has no adequate wetting and functional groups, which in turn results in coarse interface structures and causes reduction or no apparent variation of the adhesion strength. The fracture surfaces after single fiber pull-out tests exhibit adhesive interfacial failure along the fiber surface, which is further confirmed by similar adhesion strength and interlaminar shear strength values when the fiber was embedded in various epoxy resins with different temperature behavior.