The　adsorption　and　decomposition　of　ClCN　on　Si　(100)-(2　x　1)　surface　is　studied　by　using　density　functional　theory　and　the　cluster　model　method.　The　Si9H12　dimer　cluster　is　used　to　simulate　the　surface.　The　present　calculations　show　that　ClCN　may　be　adsorbed　molecularly　without　a　barrier　onto　the　surface　with　both　linear　(L1)　and　parallel　(S1)　modes.　L1　can　isomerize　to　S1　with　a　small　barrier　of　5.6kcal/mol.　Both　ClCN　and　CINC　end-on　adsorbates　were　found　to　dissociate　readily　to　produce　Cl　and　CN　species,　which　was　in　excellent　agreement　with　the　experiment.