Because　of　the　lower　total　number　and　density　of　defects　in　nanocrystals　than　those　in　their　bulk　counterparts,　the　elastic　strain　limits　and　the　plastic　deformation　behaviors　of　the　former　can　be　very　different　from　those　of　the　latter.　Furthermore,　as　the　surface　atomic　ratio　increases,　a　surface-dominant　elastic　and　plastic　deformation　characteristic　may　appear　in　nanocrystal　metals.　The　competition　between　nano-strengthening　and　surface　effects　thus　determines　the　apparent　mechanical　behaviors　of　nanocrystal　metals.　In　this　study,　we　conducted　a　series　of　in　situ　atomic-resolution　deformation　experiments　on　high　stacking　fault　energy　platinum　nanocrystals　using　an　aberration-corrected　high-resolution　transmission　electron　microscope.　From　the　direct　in　situ　atomic-scale　observations,　we　provided　direct　atomic-resolution　plastic　deformation　mechanisms　for　the　Pt　nanocrystals　of　size　ranging　from　20　to　similar　to　0.7　nm.　As　the　nanocrystal　size　decreased,　a　crossover　occurred　from　dislocation　slip-to　dislocation-free-mediated　plastic　deformation.　For　nanocrystals　of　size　above　similar　to　6　nm,　the　plastic　deformation　was　dominated　by　full　dislocation.　However,　for　nanocrystals　of　diameters　below　similar　to　2　nm,　it　was　uncovered　that　the　plastic　deformation　was　dominated　by　the　dislocation-free　plastic　deformation.　In　the　elastic　regime,　the　Pt　nanocrystals　reached　a　low　elastic　strain　plateau　by　1.5%　when　the　size　was　20　to　similar　to　9　nm.　The　elastic　strain　increased　when　the　crystal　size　was　below　similar　to　9　nm,　and　the　Pt　nanocrystals　remained　on　the　theoretical　elastic　strain　limit　plateau　by　above　similar　to　7.0%　when　the　crystal　size　was　below　similar　to　2　nm　(C)　2018　Elsevier　Ltd.　All　rights　reserved.