Nanocrystalline　metals　often　display　a　high　strength　up　to　the　gigapascal　level,　yet　they　suffer　from　poor　plasticity.　Previous　studies　have　shown　that　the　development　of　hetero-sized　grains　can　efficiently　overcome　the　strength-ductility　trade-off　of　nanocrystalline　metals.　However,　whether　this　strategy　can　lead　to　the　fabrication　of　nanocrystalline　nanowires　exhibiting　both　high　strength　and　superplasticity　is　unclear,　similar　to　the　atomistic　deformation　mechanism.　In　this　paper,　we　show　that　ultra-small　nanocrystalline　Au　nanowires　comprising　grains　in　both　the　Hall-Petch　and　inverse　Hall-Petch　grain-size　regions　can　exhibit　extremely　high　uniform　elongation　(236%)　and　high　strength　(2.34　gigapascals)　at　room　temperature.　In　situ　atomic-scale　observations　revealed　that　the　plastic　deformation　underwent　two　stages.　In　the　first　stage,　the　super-elongation　ability　originated　from　the　intergrain　plasticity　of　small　grains　via　mechanisms　such　as　grain　boundary　migration　and　grain　rotation.　This　intergrain　plasticity　caused　the　grains　in　the　heterogeneous-structured　nanowires　to　grow　very　large.　In　the　second　stage,　the　super　elongation　ability　originated　from　intragrain　plasticity　accompanied　by　the　diffusion　of　surface　atoms.　Our　results　show　that　the　hetero-grain-sized　nanocrystalline　nanowires,　comprising　grains　with　sizes　both　in　the　strongest　Hall-Petch　effect　region　and　the　inverse　Hall-Petch　effect　region,　were　simultaneously　ultra　strong　and　ductile.　They　displayed　neither　a　strength-ductility　trade-off　nor　plastic　instability.　(c)　2022　Published　by　Elsevier　Ltd　on　behalf　of　The　editorial　office　of　Journal　of　Materials　Science　&　Technology.