A simple wet chemical reduction method was developed to prepare in situ growth of Cu nanoparticles (NPs) decorated on polydopamine (PDA) functionalized graphene oxide (GO) nanosheets. The immobilized Cu NPs with diameters of 10-15 nm were uniformly distributed on PDA/GO nanosheets. The PDA layer on the surface of GO not only provides an activated platform for the uniform Cu NPs immobilization, but also remains abundant functional groups in Cu/PDA/GO nanocomposites even if after Cu NPs decorating on PDA/GO surface, which give rise to good dispersibility of Cu/PDA/GO nanocomposites in the base oil. In tests with sliding steel surfaces, the soybean oil with 0.1 wt% Cu/PDA/GO nanocomposites exhibited the lowest friction coefficient under all of the sliding conditions. Optical microscope images of the wear scar of the steel disc showed that Cu/PDA/GO nanocomposites exhibited higher antiwear capability than Cu NPs, GO and Cu/GO. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy (Rm) of the worn surface lubricated by the soybean oil with Cu/PDA/GO nanocomposites showed that formation of low shear strength tribofilms containing Cu/PDA/GO nanocomposites and its self-lubricating property was key factor in reduction of the friction and protection against wear and deformation. (C) 2017 Elsevier B.V. All rights reserved.