An amine-terminated hyperbranched polyimide (HBPI) was prepared by the condensation polymerization of a commercially available triamine monomer with a dianhydride monomer. The effects of the HBPI content on the thermal and mechanical interfacial properties of diglycidyl ether of bisphenol A (DGEBA) epoxy resins were investigated with several techniques. The thermogravimetric analysis results showed that the thermal stability of the DGEBA/HBPI blends did not obviously change as the HBPI content increased. The glass-transition temperature (T-g) of the DGEBA/HBPI blends increased with the addition of HBPI. Improvements in the critical stress intensity factor (K-IC) and impact strength of the blends were observed with the addition of HBPI. The K-IC value and impact strength were 2.5 and 2 times the values of the neat epoxy resins with only 4 wt % HBPI. The fractured surfaces were studied with scanning electron microscopy to investigate the morphology of the blends, and they showed that shear deformation occurred to prevent the propagation of cracks in the DGEBA/HBPI blends. These results indicated that a toughness improvement was achieved without a decrease in the thermal stability or T-g. (c) 2006 Wiley Periodicals, Inc.