Al-20Si　alloy　was　modified　treatment　by　P　and　RE　compound　addition,　and　the　copper　mold　was　used　to　cool　castings　with　different　thickness.　The　effects　of　composite　modification　on　Si　phase　morphology　and　the　wear　behavior　of　the　alloy　parts　were　analyzed　by　OM　and　SEM.　The　results　reveal　that　the　recalescence　temperature　of　the　alloy　is　reduced　due　to　the　high　heat　storage　coefficient　of　the　copper　mold　and　the　decrease　of　the　wall　thickness　of　the　casting,　which　is　together　with　the　composite　modification　to　refine　Si　significantly.　The　average　grain　size　of　primary　Si　in　the　castings　with　5　mm　thickness　is　only　7.1　μm.　As　the　grain　size　decrease,　its　hardness　is　increased　gradually　and　wear　rate　is　decreased.　With　the　decrease　of　castings　thickness　gradually,　the　wear　mechanism　of　alloy　is　characterized　by　from　stripping-wear　to　a　mixed　mechanism　of　abrasive　wear　and　adhesive　wear.　©　2019,　Journal　Office　of　Special　Casting　and　Nonferrous　Alloys.　All　right　reserved.