Multiwall carbon nanotubes (MWNTs) were modified by three methods, namely, oxidizing the tubes and opening both ends, filling the tubes with Ag, and grafting the tubes with hexamethylene diamine. Modified MWNTs/epoxy composites were prepared by melt-mixing epoxy resin with the tubes. Transmission electron microscope images showed that the modified MWNTs can be dispersed in the epoxy matrix homogeneously. The dielectric behaviors and mechanical properties of the composites were investigated. The dielectric and mechanical properties of the modified MWNTs/epoxy composites were considerably improved compared with those of the epoxy matrix. The tensile strengths of the Ag-filled, opened, and grafted MWNTs composites at the same filler content of 1.1 wt% were higher by similar to 30.5%, 35.6%, and 27.4%, respectively, than that of neat epoxy. The Izod notched impact strength of the grafted MWNTs/epoxy composite with filler content of 1.1 wt% was approximately four times higher than that of neat epoxy. A dielectric constant of similar to 150 of the composite with 1.1 wt% Ag-filled nanotubes was observed in the low-frequency range, which was similar to 40 times higher than that of the epoxy matrix. The proper modification of nanotubes provides a way to improve the properties of the polymer-based composites. POLYM. ENG. SCI., 2013. (c) 2012 Society of Plastics Engineers