We report on the preparation of graphite-like carbon-coated ZnO nanoparticles by ball milling a powder mixture of ZnO and C. Magnetic measurements indicated that carbon coating can strongly enhance the room-temperature ferromagnetism of ZnO nanoparticles, depending on preparation conditions. This enhancement may stem from robust ferromagnetism at the ZnO/C interface. The origin of interfacial ferromagnetism is attributed to crystallographic defects at the surface of ZnO crystals which are stabilized by the carbon shell layer.