Two kinds of Al based functionally gradient composite tubes reinforced by primary Si particles alone and primary Si/ in situ Mg2Si particles jointly were successfully prepared by centrifugal casting, and their structural and mechanical characters were compared. It is found that the composite reinforced with primary Si particles takes a characteristic of particles distribution both in the inner and outer layers. However, composite reinforced with primary Si/Mg2Si particles jointly takes a characteristic of particles distribution only in the inner layer and shows a sudden change of particles distribution across the section of inner and outer layers. The hardness and wear resistance of Al-19Si-5Mg tube in the inner layer are greatly higher than that in the other layers of Al-19Si-5Mg tube and Al-19Si tube. Theoretical analysis reveals that the existence of Mg2Si particles is the key factor to form this sudden change of gradient distribution of two kinds of particles. Because Mg2Si particles with a lower density have a higher centripetal moving velocity than primary Si particles, in a field of centrifugal force, they would collide with primary Si particles and then impel the later to move together forward to the inner layer of the tube.