The　influence　of　pulse　current　on　microstructure　and　mechanical　properties　were　investigated　for　friction　stir　welding　(FSW)　of　3　mm　thick　AZ31B　Mg　alloy　with　pulse　current　ranging　from　0A　to　400A.　The　grain　size　of　SZ　is　the　same　of　that　of　TMAZ　as　affected　by　pulse　current,　resulting　in　more　smoothie　transition　of　microstructure　from　the　SZ　to　the　HAZ.　The　ultimate　tensile　strength　and　elongation　of　EFSW　joints　both　enhanced　with　increasing　pulse　current　and　fracture　position　shifted　from　SZ　to　HAZ.　The　EFSW　joint　has　the　highest　hardness　at　pulse　current　of　300A,　where　a　large　quantity　of　precipitates　consisting　of　Al　and　Zn　elements　that　were　refined　without　coarsening　were　uniformly　distributed　in　grain　boundaries,　contributing　to　precipitation　hardening　effect,　which　is　main　strengthening　mechanism　for　enhancing　mechanical　properties　of　the　EFSW　joints.