Effect of Co content on the microstructure and damping capacity of Fe-23%Mn-X%Co alloy was studied. The volume fraction of ${varepsilon}$ martensite of the alloy was increased with increasing Co content. The hardness was increased with lowering cooling temperature and increasing Co content in Fe-23%Mn-X%Co alloy, which is ascribed to the increase in ${varepsilon}$ martensite content. The damping capacity of Fe-23%Mn-X%Co alloy was linearly increased with increasing the strain amplitude, and was constant regardless of Co content at the same volume fractions of ${varepsilon}$ martensite when the low strain amplitudes ($1{sim}3{ imes}10^{-4}$) were applied, while the damping capacity with large strain amplitudes ($4{sim}6{ imes}10^{-4}$) became higher with increasing Co content at all valume fractions of ${varepsilon}$.