Co-CoO multi-layer films were prepared using conventional sputtering method by modula ting the substrate bias voltage. The periodicity of these films with period length of 60 Å ∼ 300 Å was confirmed by X-ray small angle scattering method. For films with a period length less than 60 Å, however, X-ray diffraction peaks at low angle did not appear. Bias modulated CO-CoO films with period length of 60 Å or less show the perpendicular magnetization, though films with the same period length prepared by modulating oxygen partial pressure (Po₂) in the sputtering chamber have in-plane magnetization. Interface between Co layer and CoO layer of the bias modulated film is different from that of Po₂ modulated film. This causes the difference of their magnetic properties.