Microstructures and damping properties of semi-solid AM50 (Mg-5%Al-0.3%Mn) alloy were investigated and compared with those of die-cast AM50 alloy, based on experimental results of X-ray diffractometry (XRD), optical microscopy (OM), hardness tests and damping tests in a flexural mode. The semi-solid AM50 specimens show higher damping capacity than die-cast one in as-fabricated state, and the higher the fraction of solid α-(Mg), the greater the damping capacity. The annealing at 200oC deteriorates the damping properties of the semi-solid and die-cast specimens. This would be due to the segregation of solute atoms on dislocation lines, which eventually leads to lower internal friction by the restriction of dislocation movement. The peak damping values of the AM50 specimens are obtained after annealing at 400oC. The disappearance of segregates acting as pinning points of dislocations is thought to be responsible for the improvement in damping capacity. This result implies that the presence of solid α-(Mg) phase and annealing treatment at high temperature are beneficial to damping property of AM50 alloy.