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Due to inherent high damping capacities of Mg, Mg-based alloys or composites can absorb enormous elastic energy, thus, can be used as damping materials playing important role in reducing vibration and noise. Spherical Mg2Si magnesium matrix composites were prepared by super-gravity method with fly ash floating beads with particle sizes of 80, 125 and 500 μm. The microstructures and damping capacities of prepared samples were discussed. Spherical and fragment-shaped Mg2Si phases were observed in the matrix by SEM micrograph. Increasing particle size of fly ash floating beads can lead to an obviously decreasing tendency of internal friction tan φ. The critical strain εcr=1.9×10−2 of ε – tan φ curve and critical temperature Tcr=150 °C of T – tan φ curve for tan φ changing from slowly to greatly increasing. Damping peaks occur in the vicinity of strain ε=5.2×10−2 in ε –tan φ curve whereas no damping peaks occur in T –tan φ curve.