Ferrite nanoparticles (magnetite (Fe3O4) and cobalt-ferrite (CoxFe3-xO4)) coated with silica (SiO2) were prepared using water-in-oil microemulsion of a polyoxyethylen(15)cetylether/cyclohexene system. Observation via transmission electron microscopy revealed that the ferrite nanoparticles were located nearly at the center of spherical SiO2 particles. The sizes of Fe3O4 and CoxFe3-xO4 nanoparticles were in the range of 8-12 nm and 10-14 nm, respectively, and the thickness of the SiO2 layer was similar to14.0 nm. The solid solution of cobalt ions into Fe3O4 to form CoxFe3-xO4 nanoparticles led to an increase in the coercivity of the SiO2-coated ferrite nanoparticles. The coercivity increased with increasing amounts of added cobalt, obtaining a maximum value of 780 Oe around a Co/Fe ratio of 0.3-0.4.