The first-principles technique is employed to determine the adsorption system of the CO monolayer on Cu(100) surface under the coverages of 1.00 ML, 0.50 ML and 0.25 ML. The calculation shows that the CO dimer is not stable enough. The atomic structures of CO monolayer on virtual Cu(100) surface and in the adsorption system on Cu(100) surface are proposed. In the CO/Cu(100)adsorption system, under the coverage of 1.00 ML the top and bridge sites are both stable, but the hollow site is not stable enough;unde...