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 ∼150% uniform elongation and ∼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.