The purpose of the present work is to study the formation of the compositional gradient during the fabrication of FGMs by a centrifugal in-situ method. Al-Al2Cu FGMs were fabricated by the centrifugal in-situ method using eutectic Al-33mass%Cu alloy, and the microstructures of fabricated FGMs were studied. It was found that the Al-Al2Cu FGMs could be fabricated by the centrifugal in-situ method from eutectic alloy. Based on the experimental results, the formation mechanism of the compositional gradient during the fabrication of FGM by the centrifugal in-situ method in the A-B alloy could be summarized as follows; 1) Partial separation of A and B elements in the liquid state occurs due to the density difference. 2) A compositional gradient is formed before the crystallization of the primary crystal. 3) The primary crystals in the matrix appear to depend on local chemical composition. 4) The primary crystals migrate according to density difference, and a further compositional gradient is formed.