Hygroscopic aging in thermoset epoxy occurs inevitably as a result of the moistures uptake at various service conditions. The water molecules in epoxy diffused into hydrophilic sites cause swelling and weaken the materials’ interface in epoxy/graphene nanocomposite. This is important issue in a sense that this diffusion of water can cause fast fracture in interfacial area of the nanocomposites. Thus, we investigate the diffusion of water in epoxy and interfacial properties of epoxy/graphene nanocomposite by using molecular dynamics simulation. EPON862® resin and TETA curing agent are used to construct matrix phase in nanocomposite. The crosslinking ratio varies from 30 to 70% and 2wt% and 4wt% of water fraction are considered with a pristine and an oxidized graphene. On each nanocomposites structure, the diffusion coefficient of water and cohesive law at the epoxy-graphene interface are studied using classical molecular dynamics simulations.