Abstract: 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.