The　effect　of　cooling　rate,　undercooling　degree　and　recalescence　temperature　on　the　morphology　of　the　primary　Si-phase　and　the　mechanical　property　of　Al-20%Si　alloy　were　investigated　by　means　of　high-precision　thermometer,　optical　microscope　(OM)　and　scanning　electron　microscopy　(SEM).　The　results　showed　that　the　average　size　(D)　of　the　primary　Si　in　Al-20%Si　alloy　is　a　power　function　of　the　cooling　rate　(v)　as　D=260.6v-3/4,　and　linearly　related　to　the　recalescence　temperature　(Tm)　as　D=　0.25Tm-143.12;　Reducing　the　recalescence　temperature　of　the　primary　Si　growth　was　the　key　to　control　the　grain　growth,　the　copper　mold　with　high　thermal　storage　coefficient　may　be　favourable　to　the　sustainable　reduction　of　the　nucleation　temperature　and　recalescence　temperature　of　the　primary　Si,　so　that　the　primary　Si　size　was　small;　The　critical　supercooling　degree　of　70　K　was　needed　for　the　transformation　of　the　primary　Si　growth　from　facet-like　to　non-facet-like　ones,　which　is　consistent　with　the　theoretical　calculation　(74　K).　With　the　increase　of　cooling　rate　and　the　undercooling　degree,　while　the　decrease　of　recalescence　temperature,　the　solidified　microstructure　of　Al-20%Si　alloy　was　refined　remarkably,　correspondingly,　the　tensile　strength　of　the　Al-20%Si　alloy　increased　from　167　Mpa　to　210　Mpa　and　the　elongation　increased　from　2.14%　to　3.89　%　respectively.　©　All　right　reserved.