The surface modification of poly (p-phenylene-2,6-benzobisoxazole) (HMPBO) fibers by silane coupling agent of ${gamma}$-aminopropyl triethoxy silane (KH-560) treatment assisted by ultrasonic vibration was investigated. The chemical composition and surface morphologies of the HMPBO fibers were analyzed and characterized by XPS, FTIR, TGA and SEM. The tensile properties of the HMPBO fibers were also studied. The results indicated that polar hydroxyl groups were successfully introduced on the HMPBO surface after the proposed treatment processes, and the surface roughness of HMPBO fibers was increased. Moreover, the treated HMPBO maintained relatively excellent tensile strength, and the single fiber pull-out strength of HMPBO was improved from 0.94 MPa to 1.07 MPa.