An Au/Fe3O4 nanocomposite catalyst was fabricated through a simple deposition–precipitation method. The Au/Fe3O4 nanocomposite is a true nanocomposite that has single crystalline Au nanoparticles supported on single crystalline Fe3O4 nanoparticles. Lattice fringes from both Au and Fe3O4 single nanoparticles were simultaneously observed by transmission electron microscope (TEM). This nanocomposite catalyst showed much high activity in low temperature CO oxidation reaction. The Au/Fe3O4 nanocomposite catalyst reaches 100% CO conversion at 40 °C. In comparison, Au/commercial Fe3O4 catalyst needs 375 °C to convert CO. This Au/Fe3O4 nanocomposite is an ideal sample to study synergetic effect between the catalyst and the support at nanoscale.