This paper reports an effective approach for the fabrication of low-k polybenzoxazole (PBO) by covalent incorporation of hyperbranched PBO (HBPBO). Soluble o-aminophenol-terminated HBPBO was first synthesized by a one-pot polycondensation, then covalently bonded to poly(hydroxyamide) ended with carbonyl chloride groups, PBO was finally obtained by thermal conversion of the precursor polymers. It was demonstrated that the thermal stability of PBO was not influenced by the incorporation of HBPBO due to their similar chemical compositions, while the glass transition temperature, elastic modulus and hardness were improved because of the linking effects of HBPBO to linear PBO chains. The dielectric constant of PBO was significantly reduced due to the disturbed chain packing of PBO by the globular HBPBO, and the intrinsic cavities in HBPBO, which increased free volume in the materials. Moreover, the material also exhibited reduced coefficient of thermal expansion as compared to neat PBO and low water absorption. (C) 2015 Wiley Periodicals, Inc.