Tool　wear　process　is　a　complex　thermo-mechanical　coupling　dynamic　process.　Tool　wear　will　cause　the　change　of　the　dynamic　property　of　the　cutting　process,　namely　the　process　damping.　A　new　method　for　monitoring　tool　wear　by　on-line　recognition　of　damping　behavior　of　cutting　vibration　is　presented.　By　modeling　the　milling　force　of　worn　tools,　the　influence　of　tool　wear　on　milling　force　and　cutting　vibration　is　analyzed.　For　accurately　identify　the　time　varying　process　damping　with　tool　wear　increases,　the　vibration　component　near　the　dominant　mode　of　cutting　system　is　extracted　from　cutting　vibration　signal,　and　the　local　mean　damping　ratio　is　calculated　using　the　instantaneous　frequency　(IF)　and　the　instantaneous　amplitude　(IA).　The　method　performance　is　demonstrated　through　experimental　data　of　milling.　The　results　indicate　that　the　local　mean　damping　ratio　of　milling　vibration　increases　with　tool　wear,　which　can　clearly　indicate　the　wear　progression　of　tool.　The　method　provides　an　effective　monitoring　index　for　tool　wear　monitoring　in　machining.