The microstructural changes of Fe-22%Mn-12%Cr-4%Co-0.3%Ti-x%Si (mass %) damping steels have been studied during solution nitriding at 1100°C. In the solution nitriding free specimens, austenite, ferrite and ε-martensite were formed all over the specimens. However, the original microstructures to form in the solution nitriding free specimens were changed into single austenite phase by the solution nitriding and fine nitride with coherent relation with matrix precipitated in the austenite. With increasing Si contents, the amounts of coherent precipitates and stacking faults decreased and increased, respectively. It has been confirmed that Si hinders the permeation of nitrogen from the surface to the interior of specimen, suppresses the precipitation of nitrides such as TiN and AlN, and reduces the energy for the formation of stacking fault and that solution nitriding is a useful method to improve the surface properties of damping steels.