The　hot　cracking　behaviors　of　Mg-5Zn-xEr　(x　=　0.83,　1.25,　2.5,　5　wt.%)　alloys　are　investigated　by　optimized　hot　cracking　experimental　apparatus,　optical　microscope,　and　scanning　electron　microscope,　such　as　contraction　behaviors,　feeding　behaviors,　and　permeability　characteristics.　It　is　found　that　the　solid　phase　fraction　at　hot　crack　initiation　and　within　the　freezing　range　both　increased　with　increasing　Er　contents　up　to　2.5　wt.%　and　then　decreased　at　5　wt.%　Er　content.　The　Mg-5Zn-5Er　alloy　exhibits　the　lowest　solid　phase　fraction　(87.4%)　and　a　reduced　freezing　range　(74.2　degrees　C),　which　leads　to　more　effective　liquid　feeding　in　the　latter　stages　of　solidification.　Combined　with　the　grain　size,　the　permeability　of　the　mushy　zone,　and　fracture　morphology,　the　overall　permeability　is　optimal　in　the　Mg-5Zn-5Er　alloy,　which　is　beneficial　for　feeding　the　cavities　and　micro-pores.　Meanwhile,　a　large　amount　of　W　phase　precipitated　by　the　eutectic　reaction　(L　-＞　a-Mg　+　W　phase),　which　facilitates　healing　of　the　incurred　cracking.　Conversely,　the　Mg-5Zn-2.5Er　alloy　shows　inferior　feeding　ability　due　to　the　lowest　solid　phase　fraction　(98.3%),　wide　freezing　range　(199.5　degrees　C),　and　lowest　permeability.　Therefore,　the　Mg-5Zn-2.5Er　alloy　exhibits　maximal　hot　cracking　susceptibility,　and　the　Mg-5Zn-5Er　alloy　exhibits　minimal　hot　cracking　susceptibility.　This　work　provides　guidance　for　improving　the　hot　cracking　resistance　of　cast　Mg-Zn-Er　alloy　and　enables　an　understanding　of　the　hot　cracking　behaviors　of　Mg-Zn-RE　alloys.