Annealing Cu(111) covered with more than 10 ML Co film results in substrate Cu atoms diffusing through the Co film, and forming a Cu overlayer capping the Co him. Through AES and LEED experiments and QKLEED calculation, we found that the coverage of the Cu cap has an important effect on the top layers of the Co film. When the coverage of the Cu cap is less than 0.7 ML, the stacking sequence of the top layers is hcp, the same as that of Co film. When the coverage of the Cu cap is more than 1 ML, the top layers become fee and present twinned fee because of stacking on hcp Co him. This tends to explain the results of Co/Cu(111) superlattice studies in which Co show a fee stacking sequence. (C) 1997 Elsevier Science B.V.