The　behaviors　of　trace　element　Er　on　binary　Al-Mg　and　ternary　Al-Mg-Mn　alloys　have　been　investigated　through　optical　microscopy,　x-ray　diffraction,　scanning　electron　microscopy　with　energy　dispersive　spectroscopy　and　transmission　electron　microscope.　It　was　found　that　Er　additions　more　than　0.4%　(wt%)　produced　a　remarkable　refinement　in　grain　size　of　Al-Mg　and　Al-Mg-Mn　alloys　castings.　Er　can　enhance　the　tensile　strength　of　the　two　group　experimental　alloys　significantly　but　not　greatly　decrease　the　elongation　due　to　the　formation　of　many　fine　like　spherical　and　dispersive　primary　and　precipitation　Al3Er　particles　during　casting　and　heating　process.　Al3Er　particles　have　L12　crystal　structure　(space　group　Pm3m)　with　a　lattice　parameter　of　0.42119nm,　and　have　a　coherent　relationship　with　the　matrix　phase　α-Al　(mismatch　only　4.1%),　which　can　be　acted　as　the　heterogeneous　nucleus　during　solidification　to　increase　the　rate　of　nucleation,　and　the　other　hands　it　can　strongly　pin　up　dislocations　and　subgrain　boundaries　and　retard　the　recrystallization　of　alloys.