Fine-grained polycrystalline metals have a very high yield stress and excellent workability. Hence, numerous researchers are trying to develop an efficient process to obtain such materials. Our goal is to develop an efficient severe plastic deformation (SPD) process through investigating grain-refinement mechanisms in Equal Channel Angular Pressing (ECAP). In this paper, a series of molecular dynamics (MD) simulations of severe simple-shear deformations, which are ideally equivalent to SPD applied by typical ECAP processing routes, is performed using three-dimensional models that are thin and have a square shape with a periodic-boundary condition. We analyze the influences of the processing route and initial texture on the microstructural evolution. It is shown that twinning deformations are dominant under the calculated conditions, and that the structural evolution is notably affected by the relationship between the applied simple-shear direction and the characteristic crystal orientation, which can easily cause a twinning deformation. We conclude that Route A, without a rotation of the billet between processes, is the most efficient route. This is because twinning deformations along the simple-shear direction interact with the twin boundaries developed by the stress-component conjugate to the simple-shear. Furthermore, we demonstrate that the influence of the initial texture difference remains in force during multiple processes that have the same sliding plane.