CoxFe3-xO4 (CFO) is a well-known spinel-type ferrite magnetic material showing large cubic magnetic anisotropy and magnetostriction. Epitaxial films of CFO(001) grown on MgO(001) substrates exhibit extremely large perpendicular magnetic anisotropy (PMA) exceeding 10 Merg/cm3 owing to tensile stress originating from the lattice mismatch between the films and substrates. Since the lattice constant of MgO is slightly larger than half that of the cobalt-ferrite, MgO(001) is a suitable substrate for growing CFO(001) with strong PMA. It is known that antiphase boundaries (APBs) exist at the interfaces in most of the spinel ferrite films grown on MgO(001). The high-density APBs contribute to the degradation of the magnetic properties of ferrite films such as the Fe3O4/ MgO(001). Therefore, it is crucial to develop a technique to reduce the number of APBs to utilize practically CFO films with large PMA. For this purpose, we attempted the insertion of spinel-type buffer layers and investigated how the insertion layers affect the magnetic properties of CFO films.