The surfaces of high-modulus poly (p-phenylene-2, 6-benzobisoxazole) (HMPBO) fibres have been treated by a solution of polyphosphoric acid/acetic acid (PPA/AA). The single fibre pull-out strength, chemical compositions and surface morphologies of HMPBO fibres were tested and characterized by single fibre pull-out tests, contact angles, X-ray photoelectron spectroscopy, atomic force microscope, scanning electron microscopy and thermogravimetric analyzer. Results show that this method has great potential in the surface treatment of HMPBO fibres.