Abstract: Taking AZ91D magnesium alloy as matrix metal and fly ash cenosphere(FAC) with particle diameter of 45-128 μm as particle reinforcement phase, FAC/AZ91D composite containing 0-10wt% FAC was prepared by stirring cast method, and microstructure and mechanical properties of the composite were studied.The results show that FAC distributed uniformly in the composite, the composite contained some phases, such as α-Mg, Mg17Al12, Mg2Si and MgO. The hardness of the composite improved with the increase of the content of FAC. When mass fraction and average size of FAC was 6% and 60 μm respectively, the composite had the highest hardness of 82.1 HBW. The compressive strength of the composite up to peak value of 348.3 MPa, when average size and mass fraction of FAC was 90 μm and 6% respectively. The fracture mechanism of the composite was brittle fracture among which cleavage fracture made a major role. FAC was damaged during fracture process of the composite.