Abstract: Boltorn hyperbranched polyester (HBPE) was synthesized by a “one?step” method with trimethylolpropane as a core, 2,2?dimethylol propionic acid as an AB2?type monomer and p?toluenesulfonic acid as a catalyst. Poly(3?hydroxybutyrate?co?3?hydroxyvalerate) (PHBV) was modified by solution method using HBPE as modifier and chloroform as solvent, and it was characterized by differential scanning calorimetry, mechanical property test, polarization microscope, thermogravimetric analyzer, scanning electron microscope and other analytical methods. The results indicated that the glass transition of PHBV becomes more significant with an increase of HBPE content, suggesting that the addition of HBPE significantly enhances the segmental movement capability of PHBV. The degree of crystallinity of PHBV decreased from 64.77 % to 27.68 % with the addition of 4 phr of HBPE. In this case, the tensile strain at break increased by 194.04 %, tensile strength increased by 27.22 %, and the tear strength increased by 56.28 %. Moreover, the addition of HBPE increased the tensile strength of PHBV slightly but improved the toughness of PHBV significantly. HBPE can be considered as an excellent impact modifier for PHBV.