Seismic behavior of a 2/5-scale steel frame structure with added viscoelastic dampers is studied. Major emphases are placed on the dynamic characteristics of viscoelastic damper and the numerical simulations of structural response under earthquake ground motions. The experimental results clearly demonstrated the superior performance of the structure installed with the dampers under even severe earthquake excitations. Earthquake ground motions with peak accelerations of 0.60g caused no damage to the structure and dampers, while the structure without added dampers undergoes large inelastic deformations. In order to consider all the factors which affect the properties of viscoelastic dampers in the design, empirical formulae based on regression analysis using the data obtained from damper coupon tests are proposed. The reliability of using the modal strain energy method to predict the equivalent structural damping and seismic structural response is illustrated by correlating the numerical simulation with the experimental results of the viscoelastically damped structure. It is found that the dynamic behavior of structures with added viscoelastic dampers can be satisfactorily predicted by using currently available analytical tools.