Magnetite-silica nanocomposites were controllably synthesized by a sol-gel approach, using electrostatically stabilized magnetite nanoparticles as seeds. By virtue of the complexation occurring between the iron ions on the magnetic nanoparticles and the citrate groups, highly stable magnetic fluid (MF) containing well-dispersed magnetite nanoparticles can be obtained. Controlling the surface modification degree, magnetite nanoparticles charged with different amount of citrate groups can be obtained for coating process with silica through the seeded sol-gel approach, and magnetite-silica nanocomposites with morphologies including irregular clusters, core-shell particles can be synthesized. Moreover, silica-magnetite particles with well-defined core-shell structure and controllable silica thickness can be synthesized.