The　atomic-scale　deformation　dynamic　behaviors　of　Pt　nanocrystals　with　size　of　similar　to　18　nm　were　in　situ　investigated　using　our　homemade　device　in　a　high-resolution　transmission　electron　microscope.　It　was　discovered　that　the　plastic　deformation　of　the　nanosized　single　crystalline　Pt　commenced　with　dislocation　＂appreciation＂　first,　then　followed　by　a　dislocation　＂saturation＂　phenomenon.　The　magnitude　of　strain　plays　a　key　role　on　dislocation　behaviors.　At　the　early　to　medium　stage　of　deformation,　the　plastic　deformation　was　controlled　by　the　full　dislocation　activities　accompanied　by　the　formation　of　Lomer　dislocation　locks　from　reaction　of　full　dislocations.　When　the　strain　increased　to　a　significant　level,　stacking　faults　and　extended　dislocations　as　well　as　Lomer-Cottrell　locks　appeared.　The　Lomer-Cottrell　locks　can　unlock　through　transferring　into　Lomer　dislocation　locks　first,　and　then　Lomer　dislocation　locks　were　destructed　under　high　stresses.　The　very　high　density　dislocations　and　the　frequent　dislocation　reactions　through　Lomer　dislocations　and　Lomer-Cottrell　locks　may　lead　to　work　hardening　in　nanosized　Pt.