First-principles　calculations　based　on　density　functional　theory　(DFT)　and　the　generalized　gradient　approximation　(GGA)　have　been　used　to　study　the　adsorption　of　CO　molecule　on　the　perfect　and　defective　FeS2(100)　surfaces.　The　defective　Fe2S(100)　surfaces　are　caused　by　sulfur　deficiencies.　Slab　geometry　and　periodic　boundary　conditions　are　employed　with　partial　relaxations　of　atom　positions　in　calculations.　Two　molecular　orientations,　C-　and　O-down,　at　various　distinct　sites　have　been　considered.　Total　energy　calculations　indicated　that　no　matter　on　perfect　or　deficient　surfaces,　the　Fe　position　is　relatively　more　favored　than　the　S　site　with　the　predicted　binding　energies　of　120.8　kJ/mol　and　140.8　kJ/mol,　respectively.　Moreover,　CO　was　found　to　be　bound　to　Fe　atom　in　vertical　configuration.　The　analysis　of　density　of　states　and　vibrational　frequencies　before　and　after　adsorption　showed　clear　changes　of　the　C-O　bond.