The　work　aims　to　clarify　the　fracture　mechanism　of　laser　direct　melting　deposition　(LDMD)　Ti-6Al-4V　alloy　by　studying　the　effect　of　Β　grain　boundary　on　the　nucleation　or　propagation　behavior　of　cracks,　and　provide　a　theoretical　basis　for　improving　the　mechanical　properties　of　the　alloy.　The　build　was　formed　on　Ti-6Al-4V　substrate　with　LDMD　Ti-6Al-4V　alloy　powder.　Samples　were　taken　along　the　scanning　direction　of　the　build.　The　microstructures　of　the　sample　and　the　real-time　changes　of　the　microstructure　during　in-situ　tensile　test　were　studied　at　room　temperature.　The　effect　of　Β　grain　boundary　on　crack　initiation,　propagation　and　fracture　in　tensile　test　was　also　investigated,　and　the　fracture　mechanism　was　summarized.　The　microstructure　of　the　LDMD　Ti-6Al-4V　sample　was　composed　of　coarse　columnar　Β　grain　that　grew　parallel　to　the　direction　of　the　build　macroscopically.　Within　the　Β　grain,　plenty　of　lamellar　a　and　a　colonies　with　the　same　orientation　in　a　plates　were　observed.　Hole　defects　in　a　small　amount　were　also　identified.　The　tensile　sample　by　in-situ　SEM　showed　that　under　the　transverse　tensile　load,　the　initial　deformation　of　the　sample　was　formed　around　the　hole　due　to　the　stress　concentration　and　the　crack　propagated　along　the　Β　grain　boundary.　The　Β　grain　boundary　hindered　the　tensile　force　and　resulted　in　the　lower　elongation　of　the　sample.　During　the　tensile,　the　microstructure　mainly　deformed　along　phase　a　surrounding　Β　grain　boundary　and　the　hole　defects　caused　more　serious　deformation　due　to　concentration　of　stress.　There　was　an　angle　of　45°　between　the　deformation　direction　and　the　tensile　direction.　The　hole　defect　determines　the　initial　deformation　position　of　the　sample,　and　the　Β　grain　boundary　determines　the　direction　of　crack　propagation.　Since　the　direction　of　the　tensile　specimen　taken　is　perpendicular　to　the　Β　grain　boundary　and　the　sample　has　a　lower　elongation,　so　Β　grain　boundary　plays　an　important　role　in　mechanical　properties　and　fracture　mechanism　of　sample.　©　2019,　Chongqing　Wujiu　Periodicals　Press.　All　rights　reserved.