Grazing incidence X-ray diffraction measurements with 2-dimensional detector revealed the orientations of poly(ethylene oxide) (PEO) crystals in PEO thin films with and without added cellulose single nano-fibers (CSNFs). Anisotropic diffraction patterns were observed for the PEO and CSNF/PEO composite thin films. The c-axis of the PEO crystals was preferentially oriented nearly vertical to the sample surface. In addition, the angular widths of the diffraction peaks were used to evaluate the degree of crystallite orientation, which was observed to decrease with a decrease in the film thickness or an addition of CSNF. It was concluded that growth of the PEO crystals was prevented in the direction of the film thickness due to a decrease in the film thickness or the presence of the incorporated CSNFs. (C) 2014 Elsevier Ltd. All rights reserved.