Magnetic composite nanoparticles Fe3O4/Pt were prepared via ultrasonic irradiation to the mixed solution of hexachloroplatinum acid hydrate solution and (3-aminopropyl)triethoxysilane (APTES)-coated Fe3O4 nanoparticles and subsequent dropping addition of hydrozine hydrate. The morphology, microstructure, composition and magnetic properties of the Fe3O4/Pt nanoparticles were characterized by ultraviolet-visible spectroscopy (UV-Vis), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and superconducting quantum interference device (SQUID) magnetometry. Results show that Fe3O4/Pt nanoparticles obtained under the conditions possess saturation magnetization intensity of about 17.2 (A·m2)/kg, and their average diameter is about 50 nm.