The　electrically　assisted　friction　stir　welding　(EFSW)　of　5　mm　thick　AZ31B　magnesium　alloy　with　DC　current　ranged　0-200　A　has　been　investigated.　The　effect　of　current　on　microstructure　and　mechanical　properties　of　the　EFSW　joint　was　studied.　EFSW　increased　the　heat　input　by　resistance　heat,　and　minimized　the　weakly　welded　area　at　the　root　of　the　joints.　With　the　increase　of　current,　the　stir　zone　(SZ)　become　gradually　larger,　and　there　is　no　obvious　boundary　line　between　the　shoulder　affected　zone　(SAZ)　and　the　pin　affected　zone　(PAZ),　microstructure　along　the　thickness　direction　of　the　SZ　centre　is　much　more　uniform;　the　difference　in　grain　size　between　the　SZ　and　the　thermo-mechanically　affected　zone　(TMAZ)　at　the　advancing　side　(AS)　changed　from　abrupt　to　smooth.　All　the　joints　fractured　in　regions　with　the　lowest　hardness.　The　tensile　strength　of　the　joint　increased　with　increasing　current,　the　tensile　strength　of　the　EFSW　joint　with　a　current　of　200　A　is　the　highest,　but　the　hardness　of　the　weld　is　hardly　affected　by　the　current.