Stay cables of long span cable-stayed bridges are easy to vibrate under wind or wind/rain loads owning to their very low inherent damping. To install cable dampers near to the anchorages of cable has become a common practice for cable vibration control of cable-stayed bridge structures. The performance of passive linear viscous dampers has been widely studied. However, even the optimal passive device can only add a small amount of damping to the cable when attached a reasonable distance from the cable anchorage. This paper investigates the potential for improved damping using semiactive devices based on nonlinear frictional type of dampers. The equations of motion of a cable with a friction damper were derived using an assumed modes approach and the analytical solution for the motion equations was obtained. The results show that the friction damper evokes linearly decaying of free vibrations of the cable as long as the damper does not lock the cable. The equivalent modal damping ratio of cable with the friction damper is strongly amplitude dependent. Based on the characteristics of friction damper, the authors proposed a semi-active control strategy for cable control with dampers. According to the semi-active control law, the damper force has to be adjusted in proportion to the cable amplitude at damper position. The effectiveness of passive linear viscous dampers is reviewed. The response of a cable with passive and semi-active dampers is studied. The response with a semi-active damper is found to be dramatically reduced compared to the optimal passive linear viscous damper, thus demonstrating the potential benefits using a semi-active damper for absorbing cable vibratory energy.