3D Fe3O4-graphene nanocomposites were conveniently prepared via a direct hydrothermal grafting method. On the basis of the unique properties of both single-crystalline Fe3O4 and 3D chemically reduced graphene oxide, with characteristics such as ultralow density and high surface area, the as-prepared graphene-Fe3O4 nanocomposites showed high-performance microwave absorption ability and have the potential for application as advanced microwave absorbers.