Based　on　the　current　seismic　design　specification,　the　additional　damping　ratio　formula　has　been　derived　when　lead　dampers　are　mounted　to　a　bilinear　model　structure　which　can　be　simplified　as　a　single-degree-freedom　system.　The　additional　damping　ratios　under　different　seismic　levels　are　studied.　The　results　show　that　the　additional　damping　ratio　varies　with　the　increment　of　the　structural　deformation.　Before　the　structure　yields,　the　additional　damping　ratio　is　increased　at　first　and　then　decreased,　while　its　elastic　potential　energy　reduces　after　the　structure　yields.　The　additional　damping　ratio　provided　by　the　well-designed　lead　dampers　increase　with　the　increment　of　the　structural　deformation.　The　lead　damper　design　principles　and　methods　are　also　discussed.