The influence of Co on the damping capacity of Fe-23%Mn-(0 similar to 3)%Co alloys has been studied, based on experimental results from metallography, tensile tests, and vibration tests in a flexural mode. The amount of epsilon-martensite increases with the increase of Co content, resulting in the increase of damping capacity. For the same amount of epsilon-martensite, it is observed that the higher the Co content, the greater the damping capacity. The microstructural examinations reveal that the increase in length of gamma/epsilon interface in unit area is responsible for the improvement of the damping capacity in case of the same epsilon-martensite content.