Abstract: For long-span stayed cables equipped with oil dampers, the applicability of complex eigenvalue method in evaluating non-proportional modal damping ratios was verified by comparing the damping ratios obtained from complex eigenvalue with those from nonlinear free decaying vibration, The optimum design damper size was also calculated. It was found that 1) the vibration reduction effect of oil dampers weakens with the increase of cable spans; 2) the damping ratios of the first mode significantly decrease with the increase of non-dimensional parameter λ; 3) the vibration of CFRP cables with oil dampers is more weakened than those of steel cables; 4) the optimum modal damping ratios in and out of plane can be obtained by adjusting damper direction; 5) the increase of cable sag reduces the vibration reduction effect of dampers in plane but increase that effect out of plane; 6) the damper size and direction have less influence on natural frequencies of cables.