The　mortise　and　tenon　joints　are　the　main　connection　forms　used　in　ancient　timber　buildings,　and　damaged　joints　have　a　critical　effect　on　the　safety　of　a　timber　structure.　There　are　three　main　damaged　cases　of　dovetail　joints　which　are　pulling,　contraction,　and　mixing　damages.　In　this　study,　using　a　theoretical　analysis　of　the　stress　distribution　in　a　mortise　and　tenon　joint　resulted　from　the　pullout　damage,　a　theoretical　equation　for　the　resisting　moment　of　the　joint　was　proposed.　A　finite　element　model　was　used　to　simulate　the　cyclic　displacement　loading　of　a　frame　with　intact　joints　and　with　different　levels　of　pulling　and　contraction　damaged　joints.　The　results　show　that　the　moment　capacities　both　for　the　test　and　the　simulation　were　in　good　agreement　with　each　other.　The　simulation　results　also　indicated　that　there　are　no　changes　in　the　capacity　and　energy　dissipation　of　the　pulling　damaged　joint　compared　to　that　of　the　intact　joint,　and　good　seismic　performance　still　was　provided　when　the　pulling　damage　was　less　than　2/5　of　the　joint　length.　However,　the　capacity　of　the　contraction　damaged　joint　was　significantly　reduced,　and　its　seismic　performance　was　tolerably　lost.　The　seismic　performance　of　a　mixing　damaged　tenon　with　the　same　degree　of　pulling　damage　was　between　that　of　the　pulling　damaged　tenon　and　the　contraction　damaged　tenon,　and　generally,　it　was　controlled　by　the　contraction　damage.　The　friction　between　the　tenon　and　the　mortise　is　the　main　source　of　resisting　moment　and　energy　dissipation　ability.