The friction damper can be used for improving the seismic resistance of existing buildings. The damper is often installed in bracingmembers. The energy dissipation capacity of the damping systems depends on the type of the structure, the configuration of the bracingmembers, and the property of dampers. In Korea, there are numerous low- to mid-rise reinforced concrete moment frames that wereconstructed considering only gravity loads. Those frames may be vulnerable for future earthquakes. To resolve the problem, this studydeveloped a toggle bracing system with a high density friction damper. To investigate the improvement of reinforced concrete frames afterretrofit using the developed damped system, experimental tests were conducted on frame specimens with and without the dampedsystem. The results showed that the maximum strength, initial stiffness and energy dissipation capacity of the framed with the dampedsystem were much larger than those of the frame without the damped system.