The reverse microemulsion composition consisting of 37. 0% cyclohexane, 26.0% surfactant (TX-10and AEO9), 13.0% n-pentanol and 24.0% aqueous phase was investigated and chosen for the preparation of cobalt ferrite nanoparticles. Then silicon dioxide was coated onto the surface of the magnetite nanoparticles. The two kinds of nanoparticles were characterized by means of X-ray diffractometry(XRD), scanning electron microscopy (SEM),infrared spectroscopy (IR), and energy dispersion spectrometry (SEM-EDS). The SEM results indicate that both nanoparticles have narrow size distribution, less agglomeration and are in the size range of 10 - 60 nm. XRD patterns show that there is not any peak detected except for the peaks of CoFe2 O4, and imply that the coated silicon dioxide is amorphous. IR absorption spectra of the samples show the characteristic bands of Si-O-Si group and Fe-O group. SEM-EDS indicates that the molar ratio of Fe to Si is 96.11: 3.89. These results prove that a thin film of SiO2 is coated on the surface of the magnetite nanoparticles. And the characterization of cobalt ferrite nanoparticles prepared by conventional precipitation method are compared.