The effect of Mn content on the damping capacity of gamma austenitic phase for Fe-(17-28)%Mn alloys has been studied in relation to the stacking-fault energy (SFE), on the basis of the results of microstructural observations, thermodynamic calculations and vibration tests in a torsional mode. The damping capacity of gamma austenite decreases with the increase in the Mn content. The decrease in density of stacking-faults in gamma austenite and mobility of the stacking-fault boundary, on which the damping capacity of gamma phase depends, is considered to be responsible for the deterioration of damping capacity.