Dislocations are known as the source of plastic deformation in most crystalline metals. In this study, we employed a newly developed mesoscale defect dynamics model coupling dislocation dynamics (DD) and finite element modeling (FEM) to investigate mechanical behavior of metals, focusing on the role of dislocation plasticity. Using the coupled model, we could keep track of the dynamic motion of dislocations under complex loading and environmental conditions, as well as predicting macroscopic mechanical response. In contrast to continuum based phenomenological models, our model could provide a unique opportunity to investigate detailed dynamic evolution of defect microstructure, which could allow us to obtain fundamental understanding of deformation mechanism. Finally, a new computational design guidelines for enhancing mechanical properties of metal alloy will be presented especially for nano-structural metals.