It　is　usually　believed　that　the　partial　dislocation　and　deformation　twin　are　the　results　of　permanent　plasticity　in　materials.　Here,　we　present　in　situ　atomic-scale　observation　of　reversible　stacking　fault　and　deformation　twin　during　loading　and　unloading　in　nanocrystalline　Ni　under　high-resolution　transmission　electron　microscopy.　The　high　propensity　for　the　reversibility　of　the　stacking　fault　and　deformation　twin　is　due　to　the　high　stacking　fault　force　and　small　grain　size,　and　will　provide　an　understanding　at　atomistic　scale　on　the　nature　of　the　deformation　in　nanocrystalline　materials.　(C)　2010　American　Institute　of　Physics.　[doi:10.1063/1.3527976]