This study was carried out to clarify the codeposition behavior of composite coatings, composed of Zn Ni and SiO 2 nanoparticles, for the purpose of the development of an alternative process to that using a chromating technique. The cross sectional microstructure of the coating layer was studied by TEM observations together with element mapping technique. It was found that the silica nanoparticle has not uniformly dispersed in a coating film, but distributed only in the silica rich layer with about 50 nm in thickness formed beneath the surface. The silica rich layer can improve the corrosion resistance. Furthermore, this silica rich layer plays an important role for the fixation of the silane coupling agent to replace chromating process.