L 11 type Co-Pt ordered alloy films with a large uniaxial magnetic anisotropy, Ku, of the order of 107 erg cm3 were successfully fabricated at relatively low substrate temperatures of 270-390 °C using ultrahigh vacuum sputter film deposition. L 11 type ordered Co-Pt films, with the 〈111〉 direction (easy axis of magnetization) perpendicular to the film plane, were fabricated on MgO(111) single crystal substrates and glass disks. The ordered structure was formed in a wide Pt content region of 40-75 at. %, and Ku showed a maximum at around 50 at. % Pt content. The values of the order parameter S and Ku for L 11 type Co50 Pt50 films increased as the substrate temperature Ts, increased. Ku reached about 3.7× 107 erg cm3 (S=0.54) at Ts =360 °C for the single crystal films deposited on MgO(111) substrates, indicating a potential increase in Ku by enhancing the ordering. The values of Ku for polycrystalline films deposited on glass disks were smaller than those for the single crystal films on MgO(111) substrates, however, Ku reached 1.9× 107 erg cm3 at Ts =360 °C. The experimental results demonstrate the potential of L 11 type Co50 Pt50 films for use in data storage applications, because of their very high Ku, comparable to L 10 type Fe50 Pt50 films, the relatively low fabrication temperature, and good controllability of the grain orientation.