The　A356　alloy　was　modified　by　addition　of　rare　earth　erbium.　The　morphology　of　the　eutectic　silicon　phase　was　modified　by　controlling　the　erbium　addition　amount　and　the　cooling　rate.　The　effect　of　erbium　on　modification　of　silicon　phase　in　A356　eutectic　alloy　was　determined　by　optical　microscope,　SEM,　TEM　and　EDS.　The　results　reveal　that　with　the　addition　of　0.3%　erbium,　the　microstructure　of　the　eutectic　silicon　phase　was　refined　markedly,　and　the　cooling　rate　had　significant　effect　on　the　microstructure　of　the　eutectic　silicon　phase.　At　a　high　cooling　rate　(250°C/s),　the　addition　of　erbium　(0.5%)　was　found　to　promote　the　formation　of　fine　fibrous　silicon　phases.　The　mechanism　governing　the　morphology　evolution　was　also　discussed.