The　kinetics　of　precipitation　and　coarsening　of　Si-　containing　phases　and　the　microstructure　evolution　of　the　die-casting　plate　of　supersaturated　Al-20%Si　alloy　during　thermal　treatment　at　450　to　550℃　were　investigated　by　means　of　differential　scanning　calorimeter　(DSC),　scanning　electron　microscope　(SEM),　and　X-　ray　diffractometer　(XRD).　Results　show　that　due　to　the　pressure　die　casting　induced　rapid　solidification　and　modification,　the　Al-20%Si　alloy　presented　a　non-equilibrium　solidified　and　distorted　microstructure　with　fine　Si-phases　(the　average　size　of　Si　phase　grains　was　less　than　1　μm).　In　the　annealing　process　the　lattice　distortion　of　the　α-Al　matrix　was　alleviated　to　some　extent　and　the　diffusion　mechanism　related　with　the　coarsening　of　Si-phases　seemed　to　be　the　bulk　diffusion　control　(the　coarsening　index　is　approximately　3)　with　the　activation　energy　of　69.59　kJ/mol.　The　annealing　temperature　exerted　a　significant　effect　on　the　coarsening　rate　constant　but　not　on　the　coarsening　index.　In　the　initial　stage　of　annealing,　the　tensile　strength　of　the　alloy　decreased　while　the　elongation　increased　with　the　increasing　holding　time.　As　the　holding　time　longer　than　90　minutes,　the　tensile　strength　and　elongation　both　stabilized.　©　All　right　reserved.