Mg-1.2Y-1.2Ni　(at.%)　alloy　was　extruded　at　400　degrees　C　with　an　extrusion　ratio　of　16:1　and　different　rates　from　1　to　6　mm/s.　The　effect　of　extrusion　rate　on　microstructure　and　mechanical　properties　of　the　Mg-1.2Y-1.2Ni　alloy　was　systematically　investigated.　With　the　increase　of　extrusion　rate,　the　average　recrystallized　grain　size　of　Mg-1.2Y-1.2Ni　alloy　and　mean　particle　diameter　of　Mg2Ni　phase　were　increased,　while　the　density　of　geometrically　necessary　dislocation　and　the　intensity　of　the　basal　texture　were　decreased.　When　extrusion　rate　increases　from　1　to　6　mm/s,　the　tensile　yield　strength　(TYS)　of　as-extruded　Mg-1.2Y-1.2Ni　alloy　decreases　from　501　to　281　MPa,　while　the　elongation　to　failure　increases　from　1.5%　to　6.2%.　The　Mg-1.2Y-1.2Ni　alloy　extruded　at　3　mm/s　obtained　TYS　of　421　MPa,　the　ultimate　tensile　strength　(UTS)　of　440　MPa　and　elongation　to　failure　of　2.6%,　respectively,　exhibiting　comprehensive　mechanical　properties　with　relatively　good　plasticity　and　ultrahigh　strength.　The　ultrahigh　TYS　of　501　and　421　MPa　was　mainly　due　to　the　strengthening　from　ultrafine　recrystallized　grains,　high　volume　fraction　long　period　stacking　ordered　(LPSO)　phases　and　high　density　dislocations.