This talk will be basd on publication in Nature Communication 2010. Plastic deformation in submicronmeter-sized metallic crystals is well documented. However, the deformation mechanisms of sub-ten-nanometer-sized crystals remain unclear. Here, through the in-situ High Resolution TEM observations, we show that the tensile deformation behavior of sub-ten-nanometer-sized Au is drastically different from that of the bulk counterpart. In sharp contrast to the scenario that plasticity is mediated by dislocation emission from Frank-Read source and mutiplication in bulk materials, the deformation of sub-ten-nm Au proceeded by surface originated discrete partial dislocation slip events, which produced fresh discrete surface steps on the nanosized crystals. In addition to the tensile loading directions, surface termination planes could also influence the operation of two competing deformation mechanisms involving dislocation slip and twinning. Interestingly, homogeneous shear rather than dislocation slip becomes the possible plastic deformation mechanism when the sample dimension is around 2 nm. Stress-relieve caused by the surface-mediated plastic deformation is directly visualized. These findings provide direct experimental evidence to the vast amount of theoretical modeling work on deformation mechanisms of nanomaterials appeared in recent years. The central theoretical prediction that plastic deformation in sub-ten-nm crystals is dominated by surface events has been evidenced.