In this study, the effects of C and Si on damping capacity and mechanical properties of as-cast and as-rolled Fe-17%Mn alloys were investigated as a basic study for the purpose of the commercialization of the alloy. The $M_s$ temperature of ${gamma}{ ightarrow}{varepsilon}$ martensitic transformation in Fe-17%Mn alloy was decreased with increasing C and Si contents, resulting in the less volume fraction of ${varepsilon}$ martensite. The damping capacity was also decreased with increasing alloying content because of less ${varepsilon}$ amount and the reduction in mobility of the damping sources such as the stacking fault boundaries and ${gamma}/{varepsilon}$ interfaces due to the pinning effect by alloying elements. The mechanical properties of as-rolled alloys were superior to those of as-cast alloys probably because of finer ${gamma}$ grains, larger amount of ${varepsilon}$ martensite, and chemical homogeneity.