We systematically investigated the exchange anisotropy for ferromagnetic Co70Fe30 and antiferromagnetic Pt-Mn bilayer films. We focused on the relevance between the exchange bias and the composition of the Pt x Mn1-x (14 < x < 22 and 45 < x < 56 at%) films, and we successfully optimized the composition. The crystal structure of the Pt x Mn1-x films was FCC for 14 < x < 22 at% and FCT for 45 < x < 56 at% after annealing at 370 °C for 6 h. The unidirectional anisotropy constant (J k) for fcc-Pt15Mn85 (20 nm) and fct-Pt48Mn52 (20 nm) prepared under optimum conditions in composition were 0.16 and 0.20 erg cm-2, respectively. Both Pt15Mn85 and Pt48Mn52 films showed a larger unidirectional anisotropy constant (J k) than in other reports for the bilayer system that uses Pt-Mn antiferromagnetic layer. They also showed a flatter surface than that of other antiferromagnetic/ferromagnetic materials, which is suitable to avoid unexpected interlayer coupling in spin-valve structure. The obtained Pt-Mn films with a large exchange anisotropy and slight roughness are useful as an antiferromagnetic layer in spintronic applications.