As　a　result　of　increasing　energy　demands　and　accelerated　environmental　problems,　there　is　an　urgent　need　to　improve　the　thermal　efficiency　of　ultra　supercritical　(USC)　power　plants.　To　achieve　this　goal,　advanced　ultra-supercritical　(A-USC)　technologies　with　the　main　steam　temperature　of　700　similar　to　750　degrees　C　and　pressure　of　35　MPa　have　been　developed　quickly　in　recent　years.　One　of　the　most　promising　candidate　Ni-based　superalloys　for　the　main　steam　pipe　of　700　degrees　C　ultra-supercritical　coal-fired　power　plants　is　Inconel　740H,　which　is　a　modified　version　of　Inconel　740　developed　by　Special　Metals　Corp　(SMC).　Compared　with　IN740,　the　Ti/Al　ratio　in　IN740H　is　lowered　in　order　to　stabilise　the　microstructure　at　long　ageing　times.　In　addition,　the　Nb　content　is　lowered　to　improve　the　weldability.　In　this　work,　the　microstructure　evolutions,　the　nucleation　and　propagation　mechanisms　of　microcracks　in　the　nickel　base　superalloy　Inconel　740H　at　750　degrees　C　high　temperature　were　studied　by　the　self-developed　in　situ　high　temperature　tensile　stage　inside　a　SEM.　The　results　showed　that　under　the　uniaxial　tensile　stress　at　22　degrees　C　room　temperature　and　750　degrees　C　high　temperature　conditions,　the　grain　boundaries　of　Inconel　740H　alloy　are　al-ways　the　most　primary　crack　sources.　The　strength　of　grain　boundaries　is　higher　than　that　of　grains　under　the　room　temperature,　and　the　microcracks　will　be　nucleated　at　the　grains　as　well,　but　the　relative　strength　of　grain　boundaries　will　be　weaken　under　the　high　temperature,　which　makes　the　microcracks　tend　to　nucleate　at　grain　boundaries.　The　experimental　results　also　showed　that　the　influence　of　high　temperature　on　the　mechanical　properties　is　very　significant,　the　high　temperature　reducing　the　activate　energy　of　slip　and　weakening　the　strength　of　the　grain　boundaries,　so　that　more　slip　systems　activated　and　the　grain　boundaries　occurring　bending　and　sliding　deformation,　so　further　enhance　the　ability　of　plastic　deformation　of　alloy.　However,　the　reduction　of　relative　strength　of　alloy　grain　boundaries　leads　to　microcracks　nucleation　and　propagation　more　easily　from　grain　boundaries　and　lower　the　yield　strength　and　tensile　strength　of　alloy.