The effects of the thicknesses of a Cu/Ti underlayer and a Sm-Co layer on the magnetic properties and domain structure of SmCo5 perpendicular magnetization films prepared by using an ultra-high-vacuum sputtering system were investigated. By adjusting the thicknesses of both the Cu/Ti underlayer and the Sm-Co layer, the coercivity was controlled to be a moderate value, allowing the films to be used in magnetic recording media while maintaining a squareness ratio of unity. As the Sm-Co layer thickness was reduced, the magnetic cluster size decreased and the magnetization reversal process varied from wall motion mode to a rotation mode. It is suggested that the Cu-rich region in the initial growth stage of the Sm-Co layer functions as the pinning site of the magnetic domain wall and plays an important role in controlling the magnetic domain structure.