Graphene-based nanocomposites of graphene-Co, graphene-Ni, and graphene-Fe₃O₄ were synthesized via improved chemical-carbothermal reduction using graphite and nitrates as starting materials. The Co, Ni, and Fe₃O₄ nanoparticles are uniformly loaded on the surfaces of graphene nanosheets without serious folds and conglomeration. The average dimensions of the Co, Ni, and Fe₃O₄ nanoparticles attached to graphene are approximately 50, 60, and 5 nm, respectively. Subsequently, three novel types of graphene-Co/Ni/Fe₃O₄ nanocomposite-modified glassy carbon electrodes (GCEs) were fabricated, and their electrocatalytic activity for reduction of p-nitrophenol was investigated by cyclic voltammetry in phosphate buffer solution. Results show that the current values increase as the scanning rate is increased from 70 mV·s-1 to 100 mV·s-1 and that the electrochemical reactions on the surface of the graphene-Co/Ni/Fe₃O₄-modified GCEs are diffusion controlled. Compared with the bare GCE, the graphene-Co/Ni/Fe₃O₄ nanocomposite-modified GCEs display considerably higher reduction peak current, which proves that the graphene-Co/Ni/Fe₃O₄ nanocomposites possess favorable electrocatalytic ability for reduction of p-nitrophenol.