In this paper, three types of sand-casting M2052 alloys subjected to different heat treatments have been designed and prepared in order to investigate the relationship between Mn segregation and damping capacity using dynamic mechanical analysis, optical microscopy, X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy. The results show that damping capacity has a crucial dependence on the Mn segregation in as-cast M2052 alloy. The original as-cast alloy without subsequent heat treatment shows its internal friction (Q-1) is 1.52×10-2 at a strain amplitude of γ=2×10-4, while a remarkable enhancement (2.6×10-2) of Q-1 can be obtained by ageing of the as-cast alloy at 435 °C for 4 h. This is mainly ascribed to the further formation of nanoscale Mn segregation in the Mn dendrites (so-called Mn macrosegregation) by spinodal decomposition during the ageing. On the contrary, performing the additional homogenization treatment at 850 °C for 24 h prior to the ageing at 435 °C for 4 h for the as-cast M2052 alloy can result in the obvious reduction of damping capacity (only 6.5×10-3 for Q-1), which is closely associated with the distinct decrement of lattice distortion of γ'-Mn during f.c.c-f.c.t phase transformation caused by weakening of Mn segregation at the macro/nano-scale. ; Department of Mechanical Engineering