We　report　experimental　observation　of　room-temperature　superplasticity　and　the　distinct　nanosize　effect　on　the　deformation　mechanisms　of　Au　nanowires.　The　Au　nanowires　were　subjected　to　in　situ　tensile　straining　in　a　transmission　electron　microscope　by　using　a　home-made　strain　actuator,　and　a　super　large　plastic　strain　with　approximate　to　150%　uniform　elongation　and　approximate　to　260%　total　strain　were　observed　before　fracture.　The　plastic　deformation　started　through　full　dislocation　slip　and　was　followed　by　the　activities　of　stacking　fault　ribbons　(or　dissociated　full　dislocations)　that　were　generated　from　surface　sources　and　disappeared　at　the　other　end　surfaces.　With　the　reduction　of　the　diameter　of　Au　nanowires,　the　deformation　changed　to　the　twinning　mode　through　partial　dislocation　emissions　from　sample　surfaces.　The　mechanisms　behind　the　observed　phenomena　are　discussed　in　detail.　These　results　shed　light　on　the　size-controlled　plasticity　of　nano-metals.