Mg-Zn-Er　alloy　was　prepared　by　high-intensity　ultrasonic　melt　treatment.　The　effects　of　the　ultrasonic　treatment　time　and　ultrasonic　output　power　on　microstructure　and　properties　of　Mg-Zn-Er　alloy　were　investigated.　The　results　show　that　with　increase　in　the　ultrasonic　treatment　time　or　ultrasonic　output　power,　the　microstructure　of　the　as-cast　Mg-Zn-Er　alloy　transforms　from　bulky　dentrite　into　small　uniformity　rotundity　grain.　Further　prolonging　time　or　output　power　rising　will　cause　the　microstructure　to　become　coarse　when　the　ultrasonic　treatment　time　or　ultrasonic　output　power　reaches　a　fixed　value.　It　was　found　that　the　separated　discontinuous　phases　existed　along　grain　boundaries　and　a　mass　of　particulates　uniformly　dispersed　in　the　matrix　of　the　alloy.　When　the　melt　temperature,　ultrasonic　treatment　time　and　ultrasonic　output　power　are　718°C,　100　s　and　600　W,　respectively,　the　Mg-5Zn-2Er　alloy　has　small　uniform　and　roundness　grain,　and　its　tensile　strength　and　elongation　under　room　temperature　are　199.6　MPa　and　11.6%,　respectively.