Using　homemade　device,　in　situ　atomic-scale　bending　experiments　were　conducted　on　twinned　Ni　nanowires.　Many　in　situ　atomic-scale　observations　revealed　that　the　plastic　behaviours　of　twinned　Ni　nanowires　can　be　significantly　affected　by　the　magnitude　of　the　bent　strain.　In　the　early　stages　of　bending,　their　plastic　deformation　was　controlled　by　dislocations.　With　increasing　bending,　fcc-bct　phase　transitions　were　observed.　When　the　bent　strain　increased　to　a　high　level,　grain　boundary　resulting　from　lattice　distortion/collapse　were　detected.　This　study　advances　our　understanding　of　the　plasticity　mechanisms　of　twinned　Ni.　(C)　2019　Acta　Materialia　Inc.　Published　by　Elsevier　Ltd.　All　rights　reserved.