Magnetic composite nanoparticles Fe3O4/Ag were prepared by adding silver acetate into (3-aminopropyl)triethoxysilane (APTES)-coated Fe3O4 nanoparticles solution, silver ions were absorbed on the surface and formed a coordination compound with amino groups, and finally the complexed silver ions were reduced by sodium formate to generate magnetic Fe3O4/Ag composite nanoparticles. The morphology, microstructure, composition and magnetic properties of the Fe3O4/Ag nanoparticles were characterized by ultraviolet-visible spectroscopy (UV-Vis), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM) and superconducting quantum interference device (SQUID) magnetometry. And the antibacterial activities of the composites were investigated by E. coli and S. aureus as test bacterial spawn. The results show that as-obtained magnetic silver-containing nanoparticles possess a saturation magnetization (Ms) of about 2.0 (A·m2)/kg, and their average diameter is about 30 nm; meanwhile, the antibacterial tests show the composites demonstrate great antibacterial activity for restraining these two bacteria. Under controlled experiments, the number of these two bacteria id both under 0.11×103 (CFU) after 24 h test. Further test by adding more nanocomposites could sterilize nearly 100% of the E. coli. The magnetic and antibacterial nanocomposite fabrication process provides a good example for preparing novel biological antibacterial material, which is sensitively responded to external field (such as magnetic field