Permanent　structural　changes　in　pure　metals　that　are　caused　by　plastic　activity　are　normally　irreparable　after　unloading.　Because　of　the　lack　of　experimental　evidence,　it　is　unclear　whether　the　plastic　activity　can　be　repaired　as　the　size　of　the　pure　metals　decreases　to　several　nanometers;　it　is　also　unclear　how　the　metals　accommodate　the　plastic　deformation.　In　this　study,　the　in　situ　atomic-scale　loading　and　unloading　of　similar　to　2　nm　Ag　nanocrystals　was　investigated,　and　three　modes　of　plastic　deformation　were　observed:　(i)　the　phase　transition　from　the　face-centered　cubic　(fcc)　phase　to　the　hexagonal　close-packed　(hcp)　phase,　(ii)　stacking　faults,　and　(iii)　deformation　twin　nucleation.　We　show　that　all　three　modes　resulted　in　structural　changes　that　were　reparable,　and　their　generation　and　restoration　during　loading　and　unloading　were　observed　in　situ.　We　discovered　that　the　deformation　modes　of　nanosized　metals　can　be　predicted　from　the　ratio　of　the　energy　barriers　of　the　fcc-hcp　phase　transition　(Delta　gamma(H))　and　the　deformation　twin　nucleation　(Delta　gamma(T)),　which　differ　from　those　of　the　theoretical　modes　of　relatively　large-sized　metals.　The　proposed　Delta　gamma(H)/Delta　gamma(T)　criterion　provides　insights　into　the　deformation　mechanism　of　nanometals.