<p>  Y₁₇Co₈₃ and Gd₁₇Co₈₃ (at. %) alloy thin films are prepared on Cu(111) underlayers epitaxially grown on MgO(111) substrates at a substrate temperature of 500 °C by molecular beam epitaxy. The growth behavior and the film structure are investigated by in-situ reflection high-energy electron diffraction and X-ray diffraction. YCo₅ and GdCo₅ ordered alloy crystals epitaxially grow on the Cu underlayers. The epitaxial films consist of two (0001) variants whose orientations are rotated around the film normal by 30° each other. The epitaxial orientation relationships are (YCo₅ or GdCo₅)(0001)[1100] || Cu(111)[112] (type A) and (YCo₅ or GdCo₅)(0001)[1120] || Cu(111)[112] (type B). The volume ratios of two variants, V_{type A}:V_typeB, in YCo₅ and GdCo₅ films are estimated to be 65:35 and 72:28, respectively. The long-range order degrees of YCo₅ and GdCo₅ films are respectively determined to be 0.63 and 0.65. These ordered alloy films show perpendicular magnetic anisotropies reflecting the magnetocrystalline anisotropies of YCo₅ and GdCo₅ crystals.</p>