Ultrasonic　vibration-assisted　tensile　tests　of　AZ31　magnesium　(Mg)　alloy　sheets　are　carried　out　at　different　temperatures　(150　°C,　200　°C)　and　amplitudes　(0–11.2　µm).　The　influences　of　the　ultrasonic　vibration　on　the　stress-strain　responses,　microstructural　evolution,　fracture　morphology,　dynamic　recrystallization　(DRX),　and　dislocation　structure　of　the　material　during　hot　deformation　are　discussed.　The　results　show　that　both　flow　stress　and　elongation　of　the　material　are　significantly　influenced　by　the　temperature　and　amplitude.　The　application　of　ultrasonic　vibration　delays　the　occurrence　of　DRX.　However,　it　accelerates　the　continuous　dynamic　recrystallization　process,　thereby　increasing　the　percentage　of　the　DRX.　Furthermore,　an　evolution　of　the　dislocation　structure　occurs　under　the　impact　of　ultrasonic　vibration,　offering　valuable　insights　into　the　plasticity　improvement　of　the　material.　The　underlying　mechanisms　of　ultrasonic　vibration-assisted　hot　deformation　of　AZ31　Mg　alloys　can　be　described　as　below.　Ultrasonic　vibration　affects　the　motion　and　distribution　of　dislocations,　which　have　an　effect　on　the　dynamic　recrystallization　behavior,　ultimately　leading　to　changes　in　mechanical　behavior　of　the　material.　©　2021　Elsevier　B.V.