Magnetite nanoparticles were prepared by coprecipitation of Fe2+ and Fe3+ with NH4OH, and then, amino silane was coated onto the surface of the magnetite nanoparticles. Transmission electronic microscopy shows the average size of 7.5nm in diameter. Powder X-ray diffraction and electronic diffraction measurements show the spinel structure for the magnetite nanoparticles. FT-IR spectra indicate that amino silane molecules have been bound onto the surface of the magnetite nanoparticles by Fe-O-Si chemical bonds. Energy dispersive X-ray spectroscopy (SEM-/EDS) indicates atomic ratio of 96.75:3.25 for Fe:Si, implying a nearly monolayer coating of amino silane on the magnetite particle surface according to a rough calculation. By an enzyme-linked assay, it was proved that the amino silane-coated magnetite nanoparticles could significantly improve the protein immobilization.