Abstract: The Al-xSi-yMg composites containing various contents of Si and Mg were fabricated by centrifugal casting. The macrostructures and microstructures, and wear resistance of all the castings were examined. The relation of the Si/Mg contents and the formation of primary particles was analyzed. It shows that except the Al-15Si-5Mg, a reinforcement layer containing primary Si or Mg₂Si particles in the inner layer, and the un-reinforcement layer having no particles in the outer layer of the other castings are formed. The castings fabricated with various compositions have different kinds of primary particles, different relative thicknesses of reinforcement layer(k), and different particle volume fractions. The wear resistance of the Al-xSi-yMg composites is improved with the increasing particle volume fraction. The calculation results indicate that the primary Si and Mg₂Si are formed on the condition of x≥12.8＋0.49y, y≥5(15≤x≤30, y≤8); only primary Mg₂Si particles are formed on the condition of x<12.8＋0.49y, y≥5; only primary Si particles are formed on the condition of x≥12.8＋0.49y, y<5; and no particle is formed on the condition of x<12.8＋0.49y, y<5.