Abstract: The fly ash cenospheres (FAC) were added to semisolid AZ91D Mg alloy to prepare FAC/AZ91D Mg alloy composites by means of stir casting. The effect of particle size of FAC on the damping properties of this composites was investigated. The results show that the damping properties of FAC/AZ91D Mg alloy composites are better than those of matrix materials. When the content of FAC is the same, the damping properties of FAC/AZ91D Mg alloy composites rise with the increase of particle size of FAC. FAC plays a significant role in improving the damping properties of FAC/AZ91D Mg alloy composites at room temperature. The matrix nearby FAC can produce high density dislocations, which form the plastic zone. Under room temperature, the larger the particle size of FAC is, the greater the plastic zone is, which also leads to better damping properties. As the temperature increases, the damping properties of FAC/AZ91D Mg alloy composites go up rapidly. The dislocations, grain boundaries, and movement of the interfaces between FAC and matrix are the key points to enhance the damping properties.