Graphene /epoxy nanocomposites were prepared using ultrasonication and the cast molding method.Graphene was well dispersed in an epoxy matrix.The effects of graphene addition and surface modification on the mechanical properties and fracture morphologies of composites were investigated.The results indicate that the tensile and modulus of the graphene/epoxy composites increase firstly and then decrease with the increase of graphene addition,and when the content of graphene loading reaches 0.1%,the tensile strength reaches the highest value of 60.9 MPa,which is increased by 16.88%.When the content of graphene loading reaches 0.5%,the tensile modulus of the graphene/epoxy composites reaches the highest value of 2833.3 MPa,which is increased by 48.29%.The reinforcement mechanisms of the graphene/epoxy composites were discussed according to the fracture morphologies of composites.