The　Mg-12Gd-2Er-0.4Zr　(GE122K)　is　fabricated　by　double　extrusion,　and　the　effects　of　grain　refinement　and　precipitate　evolution　on　the　mechanical　performance　of　the　extruded　alloys　are　deeply　analyzed.　Scanning　electron　microscope　results　spectacle　that　some　bulky　precipitates　in　the　single　extruded　alloy　are　fragmented　to　fine　precipitates　during　double　extrusion　and　distribute　linearly　along　the　extrusion　direction　(ED).　The　fine　precipitates　exhibit　the　strongest　dispersion　strengthening　effect,　whereas　the　bulky　precipitates　act　as　crack　resources　and　decrease　the　elongation.　After　double　extrusion,　a　refined　grain　structure　with　an　average　grain　size　of　2.7　mu　m　is　achieved,　which　is　attributed　to　dynamic　recrystallization　(DRX).　Moreover,　double　extrusion　boosts　the　content　of　rare-earth　elements　into　the　matrix　by　dissolving　the　fine　precipitates.　During　aging,　extensive　beta　＇　precipitates　form　in　the　double　extruded　alloy　than　in　the　single　extrude　alloy.　As　a　result,　refined　grains　and　strong　precipitation　strengthening　make　a　significant　contribution　to　a　high　yield　strength　(YS)　of　422　+/-　1.59　MPa　with　an　elongation　(EL)　of　6.5　+/-　0.89%　in　the　double　extruded+peak-aged　alloy　(ACX2).　The　possible　strengthening　mechanisms　are　discussed,　and　it　is　found　that　grain　refinement　and　precipitate　strengthening　are　the　main　contributors　to　the　high　strength　of　the　GE122K　alloy.　(c)　2021　Elsevier.　B.V.　All　rights　reserved.