With　the　evolution　and　degradation　of　mechanical　fault,　changes　of　the　structural　inherent　characteristics　will　directly　affect　the　overall　response　of　system.　Spur　gear,　which　worked　as　the　research　object,　is　to　be　explored　on　the　changes　of　modal　parameters　under　different　damage　state.　Optimum　driving-point　mobility　and　modal　parameter　identification　is　achieved　by　comprehensive　utilization　of　experimental　modal　analysis　and　finite　element　analysis.　MAC　is　used　to　determine　the　experiment　results　is　whether　accurate　or　not.　Then　comparing　with　the　differences　of　modal　parameters,　the　preliminary　judgment　of　gear　damage　can　be　made.　According　to　the　experimental　data　of　different　gears,　theCOMAC　is　taken　to　complete　the　correlation　analysis　and　to　judge　the　degree　of　the　damage.　The　results　shows　that　COMAC　provide　an　effective　basis　for　the　identification　of　vibration　mechanism　and　vibration　characteristic　of　fault　gear.　©　(2013)　Trans　Tech　Publications,　Switzerland.