This paper presents a concurrent multiscale study for the deformation mechanism of monocrystalline copper under dynamic uniaxial tension. The multiscale simulation is based on the coupled meshless and molecular dynamic (MD) method. Using it, the size of computational model can be extended to a large dimension (in micrometer) with an atomistic resolution. The pure MD simulation is difficult to reach this microscopic dimension because the number of atoms will be too large. In this study, it has been revealed that the deformation behavior and mechanism of the copper is sensitive to its size, geometry, and loading strain rate. In addition, the Young’s modulus is found to be independent of the cross-sectional size and the strain rate range considered in this study. On the other hand, the yield stress decreases with specimen length and increases with the loading strain rate.