It is necessary to accurately estimate the dynamic characteristics of stay cables, especially damping ratios, to design cable damper for appropriate vibration mitigation of a cable-stayed bridge in service. In this study, a vibration exciter has been developed to evaluate the cable damping of a cable-stayed bridge. The feasibility of the developed system was verified through field tests on a 100m long stay cable. The damping ratios of the stay cable were estimated based on acceleration data as well as displacement data measured by digital image processing technique. The performance of the passively operated MR dampers with the different voltage inputs were investigated by comparing the estimated damping ratios.