Sphere-like Fe3O4 aggregates were solvothermally prepared with ethylene glycol, sodium acetate and FeCl3·6H2O as raw materials. The sphere-like Fe3O4 aggregates provided heterogeneous growth sites for Au nanoparticles. These were obtained by reduction of HAuCl4 by sodium citrate under mild reaction conditions and the Fe3O4-Au nanocomposites were subsequently formed. The peroxidase-like activity of nanocomposites was studied with H2O2 and 3,3′,5,5′-tetramethylbenzidine as substrates. Fe3O4-Au nanocomposites exhibited better catalytic activity than pure Fe3O4 aggregates, mainly resulting from the special electronic structure at the interfaces between the sphere-like Fe3O4 aggregates and the gold nanoparticles.