Mg　alloy　has　hexagonal　structure　and　exhibits　poor　workability　at　room　temperature,　which　is　attributed　to　the　difficulty　in　activating　a　sufficient　number　of　independent　slips　to　accommodate　the　deformation.　Twinning　plays　an　important　role　in　plastic　deformation　of　Mg　alloys　during　low　and　medium　temperature　to　accommodate　the　imposed　strain,　especially　the　strain　along　the　c-axis.　Therefore,　the　twinning　behavior　of　AZ31　Mg　alloy　during　plane　strain　compression　at　room　temperature　was　investigated　with　EBSD　in　this　work.　Rectangular　specimens　with　a　dimension　of　10　mm　in　length,　9　mm　in　width　and　7　mm　in　thickness　were　cut　from　a　hot　rolled　plate.　The　results　show　that　{1012}　twinning　is　dominant　when　the　compression　and　constraint　direction　are　parallel　to　transverse　direction　(TD)　and　rolling　direction　(RD)　of　the　plate,　respectively.　The　twinning　variant　selection　mechanism　is　dominated　by　the　Schmid　factor　(m)　along　compression　direction,　and　also　related　to　the　constraint　direction.　The　differences　of　twinning　behavior　can　be　interpreted　by　the　twinning　strain　tensor.　For　the　case　when　single　twinning　variant　occurs　within　a　grain,　the　average　twinning　strain　tensor　of　twinning　variant　in　constraint　direction　will　result　in　spreading;　while　for　the　case　there　are　two　or　more　twinning　variants　taking　place　within　a　grain,　the　average　twinning　strain　tensor　of　the　variant　with　higher　m　will　induce　spreading　in　the　constraint　direction,　and　that　with　lower　m　results　in　size　reducing　in　the　constraint　direction.　During　plane　strain　compression,　different　twinning　variants　coordinate　with　each　other,　twinning　won＇t　be　suppressed　until　the　micro-strain　in　the　constraint　direction　reaches　0.　©　All　right　reserved.