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To overcome the shortages of Al composites reinforced with a single primary Si or Mg2Si particles, a new type of gradient composites reinforced with the two particles was investigated through centrifugal casting. The structures and the properties of the composites have the obvious gradient distribution characteristics. The inner zone is the reinforced zone with a high volume fraction of primary Si and Mg2Si particles, whereas the external zone is the unreinforced zone with few or no primary Si and Mg2Si particles. Due to the high volume fraction of the complementary particles, the hardness values of the inner zone are much more than that of the external zone. The assembling mechanism analysis reveals that the massive primary Mg2Si particles are the key factor to form the gradient composites. In the centrifugal field, the relative velocity of the lower density primary Mg2Si particles is higher than that of primary Si particles. The Mg2Si particles will collide and impel primary Si particles to move more quickly towards the inner zone of tubular parts during freezing, resulting in a strong concentration of the primary Si and Mg2Si particles in the inner zone. To obtain the enough primary Mg2Si particles, the amount of Si should not be lower than 19 wt% and that of Mg not lower than 4 wt% in ternary Al-Si-Mg alloys.