Ultrasonic　vibration　was　introduced　into　the　molten　AZ91　alloy　in　this　paper.　The　effects　of　ultrasound　power　(from　0　to　900W)　on　the　microstructure　and　mechanical　properties　of　AZ91　alloy　were　investigated.　The　results　show　that　the　microstructure,　hardness　and　mechanical　properties　of　the　AZ91　alloy　have　been　improved　with　the　ultrasonic　treatment.　In　this　work,　the　mechanical　properties　of　the　alloy　reached　the　peak　value　when　the　ultrasound　power　was　600W.　Also,　the　mechanism　of　ultrasonic　vibration　in　the　AZ91　alloy　was　discussed.