Microstructures and damping properties have been investigated in Fe-23%Mn-(0~2)%Si and Fe-23%Mn-(0~2)%Co alloys, based on experimental results from metallography, X-ray diffractometry and vibration test in a flexural mode. The amount and number density of ε martensite are increased with an increase in Co content, resulting in the improvement of damping capacity. For the same ε martensite content, the higher the Co content, the greater the damping capacity. On the contrary, an addition of Si affects to decrease the amount and number density of ε martensite, giving rise to a decay of damping capacity. The decreased ε martensite content by the addition of Si would be attributed to an increase in critical driving force for the γ→ε martensitic transformation by solution hardening effect.