Abstract: One of the most effective measures for the vibration mitigation of long stay cables of large span cable stayed bridges is using mechanical dampers. In the conventional design theory for the cable damper, it is assumed that the ends of cables are fixed. However, when the span of the cable-stayed bridge becomes large and the structure becomes more flexible, the effect of the girder vibration on the cable damper performance may have to be considered. In this paper, a simplified theoretical model with cable, damper and girder is analyzed by a complex mode method. The longest cables of three cable-stayed bridges, with a span of 400m, 650m and 1000m respectively, were simulated to show the effects of the girder vibration. The analyzed results show that the girder vibration reduces the effectiveness of the damper as the cable length increases. Therefore, it is suggested that the girder vibration should be taken into account in the cable damper design for long span cable-stayed bridges.