In this study, the mechanical properties of graphene–epoxy nanocomposites were investigated using experimental tests, molecular dynamics (MD) simulation and Halpin_Tsai semi-empirical micromechanical model. To fabricate graphene/epoxy nanocomposite, specimens containing 0, 0.3, 0.5 and 0.7 weight percent (wt.%) of graphene nanoparticle (GNP), high power dispersion sonicating method and high-speed shearing were employed. Then, tensile and flexural modulus of manufactured nanocomposites were obtained, and the results illustrated that the elastic and flexural modulus of GNP/epoxy nanocomposites increased significantly until GNP was added up to 0.5 wt.% and after that, the trend of increasing elastic modulus declined which could be due to local GNP agglomerations within the nanocomposites with higher contents of filler. Furthermore, graphene, epoxy and graphene–epoxy nanocomposites were simulated by MD, and the mechanical properties of simulated graphene–epoxy nanocomposites were calculated. However, the effect of temperature and strain rate on tensile properties of graphene–epoxy nanocomposites was studied, which showed that increasing temperature decreased the strength of graphene–epoxy nanocomposites, and also increasing the strain rates led to an increase in the elastic modulus. Finally, experimental results and the Halpin_Tsai model were compared in which a good agreement between experimental and analytical results was observed.