The effects of aging and thermal cycling treatments on the damping capacity and microstructure of Cu-(47, 55, 65)%Mn alloys and Cu-65%Mn-4.5%Ni alloy have been studied using an optical microscope, a transmission electron microscope (TEM), and a cantilever type damping measuring apparatus. The maximum damping capacity appeared after aging at 400°C for 18 hours in Cu-47%Mn, 8 hours in Cu-55%Mn, 4 hours in Cu-65%Mn, and 36 hours in Cu-65%Mn-4.5%Ni alloy, respectively. Storage at 100°C slightly reduced damping capacity of these alloys, and the higher the Mn content, the smaller the amount of the degradation. This result might be ascribed to α-Mn precipitation inside microtwins in Cu-65%Mn alloy, and to the microstructural change from tweed structure to mottled structure in the other alloys. The thermal cycling treatment between room temperature (20°C) and 250°C led to an increase in damping capacity, which is probably due to the refinement of microstructure(tweed or twin).