This study investigated the preparation and mechanical performance of graphene/metal composites using Ni nanoparticles decorated graphene nanoplatelets (Ni-GPLs) as a reinforcing component in Cu matrix (Ni-GPL/Cu). Ni-GPLs consisting of well-dispersed Ni nanoparticles strongly attached on GPLs were successfully synthesized by chemically reducing Ni ions on the surface of GPLs. The Ni-GPL/Cu composites with only 0.8 vol% Ni-GPLs exhibited a significant improvement in ultimate tensile strength (UTS), being 42 % higher than that of monolithic Cu. The significant strength enhancement is attributed to the unique structure of Ni-GPLs, which was expected to generate a good dispersion and strong GPL-Cu interfacial bonding. The UTS of 0.8 vol% GPL/Cu composites was even lower than that of the monolithic Cu due to the GPL aggregates. The obtained results indicated that Ni-GPLs are novel and effective reinforcing components for greatly improving the mechanical properties of the graphene/metal composites.