Temperature and strain-amplitude dependent variations of logarithmic decrement ( δ ) in a high damping Mn-20Cu-5Ni-2Fe (at.%) alloy are compared for 4 different processing conditions. The primarily cold-rolled sample shows δ above 0.08 in a wide temperature range of about 100 °C, while solution treating the as-casted sample results in a strain-amplitude sensitive damping behavior, as high as 0.25 at the strain of 2.0 × 10 –4 . The chemical inhomogeneity in primarily cast alloy is reflected by the flattened changes of Young's modulus ( E ), broadened X-ray patterns, and the co-existence of twin plates and tweed microstructure. While the strain-amplitude sensitive damping behavior is related to the large fraction of Mn-rich γ Mn phase, the thin (011) twin plates, intersected with the traces of stacking faults, are considered to be the product of dislocation reaction at the conjunction of intersected (111) plane. Such twins may possess a low activation energy for cyclic movement, and therefore cause a stable damping behavior in the vicinity of room temperature.