Using a covalent functionalization strategy, hyperbranched poly (trimellitic anhydride-diethylene glycol) ester epoxy resin (HTDE) was grafted on multiwalled carbon nanotubes (MWCNTs). Then the HTDE grafted MWCNTs (HTDE-g-MWCNTs) were used as toughener to prepare epoxy composites (HTDE-gMWCNT/ EP). The results show that the HTDE-g-MWCNTs are homogeneously dispersed in the epoxy matrix and the tensile strength and fracture toughness of HTDE-g-MWCNT/EP composites are enhanced. The property improvements are due to the enhanced interfacial shear stress and relieved internal residual stress. The main toughening mechanisms are pull-out, breakage of HTDE-g-MWCNTs, and their bridging effect to crack, and shear failure of epoxy matrix around HTDE-g-MWCNTs. (C) 2018 Elsevier B.V. All rights reserved.