Using　the　first-principle　method　with　the　density　functional　theory　and　the　slab　model,　the　geometries　and　electronic　structures　of　the　adsorptions　of　acetylene　and　ethylene　molecules　on　Si(100)　surface　have　been　investigated.　Our　results　indicate　that　the　dimerized　model　is　the　most　stable　configuration　for　both　ethylene　and　acetylene　when　the　converge　is　0.5　ML,　while　the　end-bridge　configuration　becomes　the　most　favorable　structure　when　the　converge　increases　to　1.0　ML.　The　analyses　of　the　band　structures　show　that　the　variations　of　the　band　gap　could　be　explained　by　considering　the　coordination　environment　of　Si　atoms　of　the　dimer,　and　for　the　same　adsorption　model,　nearly　identical　band　gap　was　observed　for　both　ethylene　and　acetylene.　The　configuration　of　adsorption　and　the　converge　have　obvious　effects　on　the　value　of　the　band　gap,　as　well　as　the　origin　of　the　smallest　band　gap.　Moreover,　our　results　demonstrate　that　the　hybrid　density　functional　method　is　suitable　to　describe　the　electronic　structure　of　Si(100)　surface,　especially　for　the　end-bridge　model.