In this study, the change in microstructure during large deformation is simulated using molecular dynamics method. A polycrystalline fcc model consisting of four grains with periodic boundary condition is prepared, and a compressive external force is imposed on the model. The plastic deformation behavior, such as change in grain shape, motion of grain boundary, generation of dislocation and slip, are investigated. Initial orientation of every grain and the combination of the adjacent grains are varied, and the dependency of the crystallographic conditions on the hange in microstructure along with the stress-strain relation are investigated. As a result, rain refinement due to generation of grain boundaries revealed to strongly depend on the relation between the original crystal orientation and imposed load direction.