Influences of Bi addition on microstructures and damping capacities of Mg₂Si/Mg–Al–Si–(Bi) composites have been investigated. The Bi addition is beneficial to modify Mg₂Si phase from coarse Chinese script shape to fine polygonal shape. Even though the Bi-containing alloy has modified polygonal Mg₂Si compounds, its damping capacity shows similar level with that of the Bi-free alloy. This would be attributed to the increased solute concentration related to high solubility of Bi in Mg, which eventually offsets the damping increment in response to Mg₂Si modification.