Cobalt-modified magnetite (Co-Fe3O4) nanoparticles coated with silica (SiO2) were prepared by using a water-in-oil microemulsion consisting of polyoxyethylen(15)cetylether, cyclohexene and an aqueous solution of FeCl3 and CoCl2. It is found from TEM observation that the Co-Fe3O4 nanoparticles are located nearly at the center of spherical SiO2 particles, and that the particle size of Co-Fe3O4 is very uniform (ca. 14-17 nm). The coercivity of the nanoparticle increases from 1.17 kA/m to 58.9 kA/m with increasing amount of Co added.