Tetragonally distorted Fe_xCo_1−x alloy films are grown on Rh (001) showing a strong perpendicular magnetic anisotropy in a wide thickness and composition range. This large perpendicular magnetic anisotropy is chemical composition dependent and reaches a maximum at x=0.4. For this composition, we observed an out-of-plane easy axis of magnetization at room temperature with film thickness up to 15 ML. Our experiments show that the proper adjustment of the Fermi level (E_F) by the variation of the Fe_xCo_1−x alloy composition and the corresponding tetragonal distortion results in a large perpendicular magnetic anisotropy.