Molecular dynamics simulations are preformed to investigate the diffusion behaviors of double-layer islands on close-packed surfaces: Ag on Ag(111) and Cu on Cu(111), showing that the top layer moves mostly via concerted motions with its shape unchanged, and the top-layer atoms descend into the lower layer mainly by two-atom exchange only when they move to the verge of the lower layer. Especially, once a descent event takes place, other atoms of the top layer descend frequently at the same place in a short period. Compared to the Cu system, the dragging and the reattachment events take place much more frequently on the Ag surface and the Ag top layer shows a stronger tendency to form a compact configuration, e.g., a hexagon, by dragging one or two atoms from the lower layer, or even the Ag island can change from the initial double-layer into a three-layer structure due to upward diffusions. (c) 2005 Elsevier B.V. All rights reserved.