Theoretical modulus of three kinds of epoxy matrices, including diethylene toluene diamine (DETDA)/4,5-epoxyclyclohexyl-1,2-diglycidyldiformate(TDE85), m-phenylenediamine (MPD)/TDE85 and single-walled carbon nanotube (SWCNT) reinforced DETDA/TDE85 were investigated via molecular dynamics (MD) simulation to establish the structure-property relationships. The flexibility and mobility of molecular chains, the packing ability of cross-linked chain segment, the fraction of free volume of epoxy resins and the cohesive energy density in different epoxy systems were analyzed in detail, respectively. The MD simulation results showed that both the slight modification in the diamine structure and the introduction of SWCNTs can result in significant changes in the microstructure parameters of epoxy matrix, however, the modulus improvement mechanisms through changing curing agents and incorporating SWCNT into epoxy matrix had similarities and differences. The MD simulation method will be of great value in predicting and analyzing some performance closely related to the microstructure parameters for the different cross-linked resin system and its composite materials. (C) 2016 Elsevier B.V. All rights reserved.