Tensile　experiment　of　AZ91D　magnesium　alloy　was　carried　out　and　serrated　flow　was　apparent　throughout　the　deformation　history.　Dynamic　strain　aging　(DSA)　occurs　when　the　AZ91D　magnesium　alloy　treated　by　solid　solution　treatment　has　been　deformed　at　a　set　range　of　strain　rates　(1.11×10　-4　s　-1　to　1.67×10　-3　s　-1)　and　a　certain　range　temperatures　(248　K　to　423　K).　The　critical　plastic　strain　ε　c　was　observed　to　increase　with　increasing　strain　rates　but　decrease　with　increasing　temperature.　The　diffusing　activation　energy　of　solute　atoms　during　the　DSA　occurring　in　AZ91D　magnesium　alloy　is　140.8　kJ/mol　by　calculating,　which　is　correspondence　match　with　the　diffusing　activation　energy　of　Al　solute　atoms　in　Mg　matrix.　Therefore,　the　micro-mechanism　of　DSA　in　the　alloy　is　believed　that　the　Al　atoms　in　solid　solution　gather　around　dislocations　to　form　Cottrell　solute　atmospheres　by　vacant　diffusion　and　then　pin　the　moving　dislocations.　©　(2012)　Trans　Tech　Publications.