Microstructure　of　A356　aluminum　alloy　refined　by　A15Ti1B0.5RE　master　alloy　was　observed　by　OM　(optical　microscope),　SEM　(scanning　electronic　microscope)　and　EDAX　to　understand　the　refining　effects　of　the　master　alloy.　The　results　indicate　that　the　master　alloy　can　greatly　refine　the　grain　size　and　dendrite　in　A356　aluminum　alloy,　in　which　the　RE　exhibits　significantly　positive　role.　A1TiRE　phase　formed　in　the　master　alloy　with　RE　is　unstable　in　Al　melt　it　and　could　dissolved　quickly　into　Al　melt　to　release　RE　elements,　which　can　decrease　surface　energy　and　tendency　of　segregation　of　TiB2　phase,　increasing　nuclei　amounts.　Meanwhile,　RE　can　restrain　the　growth　of　TiAl3　phase　and　the　formation　of　acicular　TiAl3　phase　as　a　result　of　RE　easily　segregating　in　grain　boundary　and　phase　interface　of　Al　melt,　raising　the　nucleation　rate.　In　addition,　RE　can　improve　the　refining　effects　of　the　master　alloy　due　to　effectively　purifying　the　metallurgical　quality　of　the　master　alloy　and　Al　melt.