Eilects　ol　varied　ultrasonic　powers　and　time　on　microstructure　and　properties　of　AZ31　magnesium　alloy　were　investigated　by　introducing　ultrasonic　into　the　melt,　and　refining　mechanism　of　ultrasonic　on　grain　size　in　the　AZ31　magnesium　alloy　was　described.　The　results　show　that　grain　size　in　the　alloy　is　obviously　refined　with　the　more　uniform　distribution　as　a　result　of　comprehensive　role　of　different　effects　from　ultrasonic,　leading　to　the　improvement　of　mechanical　properties.　Microstructure　and　mechanical　properties　of　the　alloy　are　closely　related　to　the　ultrasonic　power　and　time.　With　increasing　in　ultrasonic　power　and　time,　grain　size　is　decreased　and　tensile　strength　is　in　creased.　However,　there　exists　a　critical　value　for　ultrasonic　power　and　time.　When　ultrasonic　power　and　time　exceed　the　critical　value,　the　tensile　strength　is　decreased　with　the　slowly　fining　tendency　of　grain　size.　Desirable　grain　size　and　tensile　strength　can　be　observed　with　ultrasonic　power　of　600W　for　100　s.