According　to　the　gear　meshing　theory,　the　tooth　surface　equation　of　orthogonal　face　gear　is　derived　and　the　mathematical　model　is　established.　Also　the　model　was　provided　for　simulating　the　bevel　gear　transmission　system　concerning　the　time　variant　stiffness　and　face　gears　errors　under　static　load　and　cyclic　loads.　Through　the　model　the　computerized　analysis　of　speed,　angle　and　acceleration　of　gear　real　tooth　surface　could　be　accomplished.　Rattle　as　discussed　under　condition　of　different　static　loads　and　the　constant　cyclic　loads.