Density　functional　theory　periodic　slab　calculations　were　carried　out　for　CO　adsorption　on　a　series　of　Mo　modified　Pt(111)　surfaces　to　provide　an　insight　into　the　interaction　between　CO　and　doped　metal　surface,　an　important　issue　in　CO　oxidation　as　well　as　in　promotion　and　poisoning　effects　of　catalysis.　The　modification　of　adsorption　properties　with　respect　to　those　of　adsorption　on　the　pure　Mo(110)　and　Pt(111)　is　described　in　terms　of　changes　in　the　adsorption　energies,　adsorption　sites　and　vibrational　properties　occurring　upon　alloying.　We　believe　that　the　present　DFT　calculations　can　provide　important　information　into　optimal　alloy　composition　for　CO-tolerance,　which　is　not　easily　obtained　by　experimental　methods.