Titanium　alloy　was　extensively　used　in　aerospace　industrial　field.　It　has　a　great　significance　to　directly　reveal　the　deformation　process　as　a　function　of　application　conditions.　This　work　investigates　the　real-time　deformation　behavior　of　laser　direct　metal　deposited　(LDMD)　Ti-6Al-4V　alloy　at　200　degrees　C,　using　self-developed　high　temperature　in　situ　scanning　electron　microscope　(SEM)　tensile　stage　with　a　controlled　heating　system.　The　results　showed　that　the　microstructure　of　the　sample　comprised　of　columnar　beta　grains　with　zigzag　grain　boundaries　decorated　by　tilted　alpha　plates　along　the　building　direction.　During　uniaxial　tensile,　slip　bands　develop　along　the　grain　boundaries　between　alpha　plates　with　increasing　stress.　Grain　boundary　sliding　contributes　to　the　high　tensile　elongation.　Compared　the　mechanical　properties　along　transvers　tensile　with　those　at　room　temperature,　the　tensile　stress　decreased　and　displacement　increased　at　200　degrees　C.　Therefore,　higher　elongation　and　reduction　in　cross-sectional　area　were　identified　at　200　degrees　C　than　those　at　room　temperature.　The　fracture　of　specimen　at　200　degrees　C　shows　obvious　necking　character　and　deep　dimples　with　thin　rims.　Coalescence　of　slip　bands　at　alpha/alpha　interface　creates　voids　and　the　coupling　of　voids　nucleates　micro-crack.　alpha/alpha　grain　boundary　interface　constitutes　path　to　the　crack　propagation.　The　high　misorientation　angle　at　columnar　beta　grain　boundary　hinders　the　cracks　propagation.　Finally,　the　cracks　accumulate　along　the　columnar　beta　grain　boundary　leading　to　fracture.　(C)　2019　Elsevier　B.V.　All　rights　reserved.