Fe3O4 nanoparticles having a size of 10 nm were synthesized by the co-precipitation of Fe3+ and Fe2+ (molar ratio, 2:1) with NH4OH under a nitrogen atmosphere. The γ-Fe2O3 nanoparticles were also prepared by the sol-gel process using Fe(NO3)3ㆍ9H2O and ethylene glycol. The particle size of γ-Fe2O3 decreased upon decreasing of molar ratio R [Fe(NO3)3ㆍ9H2O/ethylene glycol]; the mean particle size was ca. 15~20 nm. Silica particles were coated on the surface of Fe3O4 and γ-Fe2O3 nanoparticles by the hydrolysis of TEOS, and then 3~APTES, an amino-silane coupling agent, was covalently coupled to the surface of the silica-coated magnetic nanoparticles. The characteristics of the silica-coated magnetic nanoparticles were analyzed by TEM, X-ray diffraction, TGA-DTA, and FT-IR spectroscopy. As a result, the structure of the magnetic nanoparticles was determined to be of the spinel type; the crystalline phase transition temperature from α-Fe2O3 to γ-Fe2O3 was ca. 400 ℃. In addition, Fe-O-Si chemical bonds appeared on the surface of the magnetic nanoparticles.