Abstract: Silica was coated on the surface of magnetite nanoparticles by the sol-gel method and alkali catalyzing hydrolysis of tetraethyl orthosilicate（TEOS）. Inversion biomicroscope，field emission transmission electron microscope，X-ray diffraction，laser particle size analyzer  and  vibrating sample magnetometry were used to characterize the appearance，size and size distribution，magnetic characteristics，crystal structure and chemical composition of the composite. The results show that the particles  have good performances, the diameter  is about 15 nm，and the magnetic property is 10.9 emu/g. The silica coated magnetite nanoparticles were used to isolate plasmid DNA and genomic DNA.