Benzene　adsorption　on　Au(100)-3x3　surface　has　been　studied　with　periodical　slab　model　by　Vosko-Wilk-Nusair　LDA　functional　under　double　numeric　basis　with　polarization　functions.　The　results　of　geometry　optimization　indicate　that　the　hollow　site　in　the　highest　coordination　is　energetically　active　for　benzene　adsorbed　on　Au(100)　surface,　and　the　adsorption　energy　is　-184.8　similar　to-184.3　kJ　(.)　mol(-1).　The　benzene　ring　has　been　distorted,　and　the　C-C　bond　has　a　considerable　elongation.　Two　opposite　carbon　atoms　are　involved　in　the　adsorption　process　leading　to　a　rehybridization　of　the　two　carbon　atoms　from　sp(2)　to　sp(3),　which　makes　the　adsorbate　state　intermediate　between　benzene　and　1,4-cyclohexadiene.　The　benzene　adsorbed　in　the　bridge　and　top　positions　is　found　to　be　energetically　less　favorable,　and　the　adsorption　energies　are　respectively　-156.7　similar　to-145.3　kJ　(.)　mol(-1)　and　-116.5　similar　to-117.0　kJ　(.)　mol(-1).　The　geometry　of　benzene　has　a　slight　elongation.　The　molecular　orbital　analysis　also　indicates　that　the　degenerate　frontier　molecular　orbitals　lose　their　degeneracy　with　decrease　of　symmetry.　The　LUMO　of　benzene　has　a　good　overlap　with　the　d(z2)　orbital　of　Au　atom.　The　two　carbon　atoms　in　the　opposite　positions　are　bonded　to　the　near　An　atoms　with　di-sigma-bonds.