Nanostructured metals have attracted progressively more attention owing to their remarkable mechanical properties of high strength, good ductility. Since grain boundaries tend to play a key role in plastic deformation in nanostructured metals, extensive studies have been undertaken to understand the role of interface structure on dislocation plasticity. However, the understanding of the underlying mechanisms that control the properties of nanostructured metals is still unresolved. In general, the grain boundaries are believed to obstruct dislocation activity as a barrier for dislocation motion at the bulk scale. Our atomistic models for bi-crystalline nanopillars have shown that dislocations would nucleate at the grain boundary at lower stress than nucleation stress in single crystalline counterparts, which could imply that the grain boundary have the potential to be nucleation sites. We also have performed dislocation dynamics simulation to investigate the size effect in bi-crystalline FCC micropillars, taking account for the role of the grain boundary as both a source and a sink. Our simulation results show the smaller size effect in bi-crystalline micropillars, showing hardening for larger samples, softening in smaller samples due to the existence of grain boundary.