Abstract: When the cenosphere fly ash content is lower, the melt is easily delaminated during preparing the cenosphere fly ash reinforced aluminium composites by stir casting technology. In order to realize the distribution of cenosphere fly ash in the aluminium melt, the different part s of the sample were analyzed by SEM. The result s indicatethat there is a pure aluminium layer in the bottom of the sample, but the cenosphere fly ash dist ributes evenly in thetop of the aluminium melt . The velocity of the cenosphere fly ash moving to the top of the melt was determined bythe difference of density between cenosphere fly ash and aluminium, diameter and volume f raction of particles, aswell as viscosity of the aluminium melt . To reduce the pure aluminium layer, the viscosity of the aluminium meltshould be increased or the time of casting process should be reduced for the certain material. The down castingmethod and quick chiller are employed, so that the cenosphere fly ash has no enough time to move upward and solidify in the aluminium melt, and the cenosphere fly ash dist ributes evenly in the aluminium melt .