By high-temperature annealing of Si(100) samples containing ion-implanted Al atoms，thermodynamic behaviors of the segregated Al atoms on the surfaces have been investigated. Experiments of annealing the samples at 900℃ show that Al and Si atoms conbine to form Al-Si clusters with size of 2—3nm，while segregated Al atoms form epitaxial Al film and islands on Si(100) surfaces. Further annealing at 1200℃ indicates that rapid cooling of samples leads to formation of cubic Al4Si grains of 20—30nm size. The Al-Si clusters seem to be independent of the substrate structure and tend to come together，which are probably the precursor of the Al-Si alloy formed in the process of the high-temperature annealing and rapid cooling.