In　situ　tensile　tests　of　Cu　single　crystalline　nanowires　in　a　high-resolution　transmission　electron　microscope　reveal　a　novel　effect　of　sample　dimensions　on　plasticity　mechanisms.　When　the　single　crystalline　nanowire　size　was　reduced　to　＜similar　to　150　nm,　the　normal　full　dislocation　slip　was　taken　over　by　partial　dislocation　mediated　plasticity　(PDMP).　For　the　first　time,　we　demonstrate　this　transition　in　a　quantitative　manner　by　assessing　the　relative　contributions　to　plastic　strain　from　PDMP　and　full　dislocations.　The　crossover　sample　size　is　consistent,　well　within　model　predictions.　This　discovery　represents　yet　another　＂sample　size　effect＂,　beyond　other　reported　influence　of　sample　dimensions　on　the　mechanical　behavior　of　metals,　such　as　dislocation　starvation　or　source　truncation,　and　the　＂smaller　is　stronger＂　trend.