When the part size is scaled down to micro-scale, the material consists of only a few grains and the material properties and deformation behaviors are quite different from the conventional ones in macro-scale. In micro-scaled plastic deformation process such as ultra-thin strip rolling, material thickness effect is difficult to reveal and investigate using conventional material models. The distributions of the stress, strain and active slip systems, and the slip and deformation behavior in rolled pure ultra-thin copper strip with the same reduction were simulated by the crystal plasticity finite element method (CPFEM) and Voronoi polycrystalline model with respect to specimen dimension, grain size, grain orientation and its distribution to evaluate quantitatively the influence of grain orientation and structure on inhomogeneous deformation behavior of ultra-thin strip rolling on a mesoscale. A polycrystalline aggregate model is generated and a crystal plasticity based an implicit finite element model is developed for each grain and the specimen as a whole. The crystal plasticity model itself is rate dependent and accounts for local dissipative hardening effects and the original orientation of each grain was generated based on the orientation distribution function (ODF). Voronoi tessellation has been applied to describe the polycrystalline aggregation. The accuracy of the developed CPFEM model is verified by the fact that the simulated stress-strain curves agree well with the experimental results. The deformation behaviors, including inhomogeneous material flow, and slip system activity with the increase of thickness size for the constant size of grain, are studied. It is revealed that when the ultra-thin strips are composed of only a few grains through thickness direction, the grains with different size, shapes and orientations are unevenly distributed in the ultra-thin strip and each grain plays a significant role in micro-scale plastic deformation, slip system activity and leads to inhomogeneous deformation. The simulation result reveals that the deformation behavior in the polycrystalline aggregates is inhomogeneous not only in intracrystalline but also in intergranule regions by simulation of deformation behavior of pure ultra-thin copper strip rolling with 40% rolling reduction. This can be attributed to the different initial grain orientations and structures, the misorientation of neighboring grains, and the different properties of active slip systems and lattice rotation. Activation often initially occurs at free surface and near the boundary adjacent to grains, and then the slip develops in interior grain. The results from the proposed modeling methodologies provide a basic for understanding and further exploring of micro-scaled plastic deformation behavior in ultra-thin strip rolling process.