The　effects　of　grain　size　on　tensile　behavior　of　Inconel718　superalloy　have　been　characterized　by　an　in-situ　tensile　stage　inside　a　scanning　electron　microscopy　(SEM)　combined　with　electron　backscatter　diffraction　(EBSD)　and　digital　image　correlation　(DIC)　at　room　temperature.　The　microstructure　evolution　and　mechanical　properties　of　specimens　under　different　heat　treatments　were　performed　and　compared.　The　in-situ　tensile　test　showed　that　the　yield　strength,　ultimate　tensile　strength　and　reduction　of　cross　section　decrease　with　the　increasing　grain　size.　An　in-situ　EBSD　study　showed　that　with　increasing　stress　the　local　plastic　deformation　become　more　inhomogeneous.　An　analysis　combined　in-situ　SEM　morphology　evolution,　DIC　strain　distribution　and　EBSD　results　indicated　that　the　grains　deformed　coordinately.　Geometrically　necessary　dislocation　(GND)　density　depends　on　the　grain　size　and　the　orientation　of　individual　grains.　The　coordinated　deformation　ability　decreased　with　increasing　grain　size.　The　fracture　morphology　showed　that　with　the　increase　of　grain　size,　the　fracture　mechanism　transformed　from　the　ductile　transgranular　fracture　to　the　brittle　intergranular-transgranular　mixed　fracture.