Structural transition of 3/8 monolayer Sn-adsorbed Cu͑001͒ surface alloy and the changes in the Sn-induced surface resonance band have been studied by low-energy electron diffraction and angle-resolved photoemission spectroscopy. The surface structure reversibly changes between ͑ −4 2 0 4 ͒ periodicity at low temperature and ͑1 ϫ 1͒ periodicity at high temperature. This transition is interpreted as an order-disorder transition from the structure model while the resonance band of the 3/8-ML

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... ͑001͒ surface significantly changes with the transition. The band folds back with an energy gap with decreasing temperature through the transition temperature. The backfolding point of the band is close to the Fermi wave number in reciprocal space in the angle range between 0°and 11°from the ⌫ M line, and is independent of surface Brillouin-zone boundary.