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Co50Pt50 (at. %) alloy thin films are prepared on MgO(111), SrTiO3(111), and Al2O3(0001) single-crystal substrates by ultra-high vacuum radio-frequency magnetron sputtering by varying the substrate temperature from room temperature to 600°C . Effects of substrate material and substrate temperature on the crystallographic and magnetic properties are investigated. CoPt-alloy epitaxial thin films with the close-packed plane parallel to the substrate surface are obtained on MgO and SrTiO3 substrates. On Al2O3 substrates, CoPt epitaxial films are obtained at temperatures higher than 500°C, whereas CoPt films consisting of epitaxial and polycrystalline crystals are formed below 400°C. From the view points of order degree and stacking sequence of close-packed plane along the perpendicular direction, the crystal structure is determined. L11 ordered phase is preferentially formed and is stabilized around 300°C for all the employed substrate materials. A higher order degree is observed for the films prepared on MgO >; SrTiO3 >; Al2O3. The magnetic properties are influenced by the crystal structure and the order degree.