The deformation of a nano-sized polycrystalline Al bar under the action of vice plates is studied using molec- ular dynamics simulation. Two grain sizes are considered; deformation in the fine-grained sample is mainly caused by grain-boundary processes (sliding and rotation), while dislocation plasticity dominates in the coarse- grained sample. We show that the sample distortion is reflected by the center-of-mass motion of the grains. Grain rotation is responsible for surface roughening after the loading process. While the plastic deformation is caused by the loading process, grain rearrangements under load release also contribute considerably to the final sample distortion.