This　study　presents　a　detailed　investigation　into　the　deformation　behaviour　and　the　factors　affecting　the　strength　and　ductility　of　laser　powder　bed　fusion　(LPBF)　Ni-based　superalloy　718　(Inconel　718)　in　both　as-printed　and　heat-treated　conditions　using　in-situ　SEM　+　EBSD.　The　results　show　that　following　post-deposition　high　homogenization　temperatures,　the　columnar　grain　structure　of　the　as-printed　Inconel　718　alloy　successfully　transformed　to　a　nearly　uniform　grain　size　and　the　subsequent　aging　processes　facilitated　the　precipitation　of　strengthening　phases　(gamma　＇,　＇　,　gamma＂).　The　specimen　subjected　to　heat　treatment　(HT1)　exhibited　higher　strength　but　lower　ductility,　while　the　as-printed　specimen　shows　higher　tensile　elongation　due　to　high　initial　dislocation　density.　In　contrast,　the　heat-treated　(HT2)　specimen　presented　a　more　optimal　combination　of　strength　and　ductility.　The　underlying　mechanism　for　the　enhanced　tensile　properties　in　the　HT2　specimen　was　nearly　homogeneous　deformation　trend　across　the　grains　and　ultrafine　precipitation　of　hardening　phases.　In　addition,　the　twin　boundaries　were　stable　in　the　early　deformation　stages　and　maintained　their　orientation.　Multiple　slip　systems　were　activated　at　the　high-deformation　stage,　resulting　in　a　high　proportion　of　geometric　necessary　dislocations.　Further,　the　interaction　of　deformation-induced　dislocation　with　twin　boundaries　transformed　them　into　general　boundaries　such　as　high　and　low-angle　grain　boundaries,　ultimately　increasing　ductility.　On　contrary,　in　the　HT1　specimen,　the　deformation　inhomogeneity　in　grains　enabled　the　strain　localization　at　grain　boundaries,　which　ultimately　caused　the　early　formation　of　cracks.