In seismic design of cable-stayed bridges, evaluation of damping of structure is important. However, because it is difficult to evaluate damping property theoretically, it is not examined enough on determination of a damping factor. In order to investigate damping property of long-span cable-stayed bridges, the ratio accounting for strain energy of each member, the effect of frictional damping at movable supports and energy dissipation from foundations are examined referring measured data on damping ratio. Influence of setting of damping ratio in seismic response analysis was also examined.