Abstract: The effect of predeformation on damping capacity of Fe19Mn and Fe19Mn1.5Si alloys was studied. The damping capacity was measured using reversal torsion pendulum with free decay method. The stacking fault probability was determined by means of XRD (Xray diffraction). The microstructure was observed using OM (optical microscope). The results indicate that with the tensile strain increasing (0%-10%), the stacking fault probability in γaustenite and εmartensite of Fe19Mn alloy increases continually. When the tensile strain is above 4%, however, Shockley partial dislocations are cut into several segments because of the reciprocal chiasm of stacking faults and perfect dislocations gliding, so the distance of two strong points decreases. The damping capacity of Fe19Mn alloy, therefore, is improved with the tensile strain increasing until it reaches maximum at 4%, and then decreases. But it is not the same regularity to Fe19Mn1.5Si alloy when tensile strain is above 4%. The εmartensite and stacking faults are small before deformation, so the predeformation can not change the microstructure of Fe19Mn1.5Si alloy and the damping capacity does not decrease when tensile strain is above 4%, but is still increasing stably.