Cu-P　master　alloy　and　Al-RE　master　alloy　were　added　into　Al-20Si　alloy　to　understand　influence　of　modification　treatment　on　microstructure　of　the　alloy　by　SEM,　X-ray　diffractometer　and　optical　metallographic　microscope.　The　experimental　results　reveal　that　the　AlP　phase　can　be　generated　by　reaction　of　P　and　Al　as　the　heterogeneous　nuclei　of　the　primary　silicon　phase　to　effectively　refine　the　primary　silicon　in　the　alloy.　The　rare　earth　elements　are　adsorbed　and　dissolved　into　the　growth　step　of　the　Si　phase,　resulting　in　the　increase　of　lattice　constant　of　Si　phase　in　the　Al-20Si　alloy　and　the　shift　of　X-ray　diffraction　pattern　with　a　small　angle.　With　the　increase　of　modification　time,　the　average　grain　size　of　primary　silicon　is　decreased　from　85.6　μm　to　13.2　μm.　The　eutectic　silicon　phase　is　changed　from　long　needle　to　short　rod　shape　with　uniform　distribution　in　the　matrix.　The　hardness(HB)is　increased　from　68.5　to　80.8.　©　2017,　Journal　Office　of　Special　Casting　and　Nonferrous　Alloys.　All　right　reserved.