Metallic　nanomaterials　are　widely　used　in　micro/nanodevices.　However,　the　mechanically　driven　microstructure　evolution　in　these　nanomaterials　is　not　clearly　understood,　particularly　when　large　stress　and　strain　gradients　are　present.　Here,　we　report　the　in　situ　bending　experiment　of　Ni　nanowires　containing　nanoscale　twin　lamellae　using　high-resolution　transmission　electron　microscopy.　We　found　that　the　large,　localized　bending　deformation　of　Ni　nanowires　initially　resulted　in　the　formation　of　a　low-angle　tilt　grain　boundary　(GB),　consisting　of　randomly　distributed　dislocations　in　a　diffuse　GB　layer.　Further　bending　intensified　the　local　plastic　deformation　and　thus　led　to　the　severe　distortion　and　collapse　of　local　lattice　domains　in　the　GB　region,　thereby　transforming　a　low-angle　GB　to　a　high-angle　GB.　Atomistic　simulations,　coupled　with　in　situ　atomic-scale　imaging,　unravelled　the　roles　of　bending-induced　strain　gradients　and　associated　geometrically　necessary　dislocations　in　GB　formation.　These　results　offer　a　valuable　understanding　of　the　mechanically　driven　microstructure　changes　in　metallic　nanomaterials　through　GB　formation.　The　work　also　has　implications　for　refining　the　grains　in　bulk　nanocrystalline　materials.