A virtual X-ray powder diffraction experiment is conducted on a realistic computer-generated nano-polycrystalline microstructure. It is shown that the size and strain broadening contributions to the diffraction line profiles can be directly and reliably extracted from the atomistic model. It is also shown that current line profile analysis methods cannot fully interpret the observed patterns due to the peculiar microstructure of grain boundaries. (C) 2012 Elsevier B. V. All rights reserved.