Identification　of　key　geometric　errors　is　an　essential　prerequisite　for　improving　the　machining　accuracy　of　five-axis　machine　tools.　This　paper　presents　a　new　sensitivity　analysis　(SA)　method　to　extract　key　geometric　errors,　and　then　to　improve　the　machining　performance　of　machine　tools　by　compensating　key　geometric　error　components.　Development　of　geometric　error　prediction　model　is　involved　to　obtain　geometric　error　values　at　arbitrary　positions　at　first.　Based　on　the　multi-body　system　theory　and　flank　milling　theory,　the　machining　error　model　is　developed,　which　considers　37　geometric　errors.　Then,　a　new　SA　method　is　introduced　by　taking　the　machining　error　model　as　sensitivity　analysis　model　and　taking　the　geometric　errors　as　analytical　factors.　Meanwhile,　a　sensitivity　index,　which　has　the　characteristics　of　simple　expression　and　clear　physical　meaning,　is　proposed,　i.e.,　the　peak　value　of　the　machining　error　caused　by　each　geometric　error.　Moreover,　the　simulations　analysis　is　carried　out　to　obtain　the　sensitivity　coefficient　of　each　geometric　error　and　the　key　error　components.　Finally,　the　validity　and　correctness　of　the　proposed　method　are　demonstrated　by　the　experiments.　Furthermore,　the　SA　method　can　be　extended　to　multi-axis　machine　tools.