The influence of surface stress on the yield strength of nanotwinned polycrystal face-centered-cubic (FCC) metallic nanowire is theoretically investigated. The contribution of surface boundaries on the strengthening/softening is analyzed in the framework of continuum mechanics theory by accounting for the surface energy effects. The other strengthening mechanisms originated from the inner boundaries are described by the Taylor model for the nanotwinned polycrystalline metals. The theoretical results demonstrate that the yield strength of nanotwinned polycrystal wires is dependent on the twin spacing, grain size and the geometrical size of the wire. The surface stress effects on the strength perform more and more significantly with decreasing the wire diameter, especially for the diameter smaller than 20 nm. In addition, the dependence of surface stress on the strength is also relevant to the size of microstructures as well as the magnitude and direction of surface stress. These results may be useful in evaluating the size-dependent mechanical performance of nanostructured materials.