•PEO/CNCs composites were prepared by solution casting technique.•The composites were characterized by SEM, FTIR, XRD, DSC, TGA, tensile testing.•Morphology, crystalline structure, and reinforcing effect have been studied.•Some composites properties are extreme at a certain PEO/CNCs ratio. Composite films of polyethylene oxide/cellulose nanocrystals (PEO/CNCs) comprising up to 75wt.% CNCs were obtained via solution casting. For the first time, investigations of the PEO/CNCs composite films over a wide composition range were carried out. Morphology, crystalline structure, thermal stability and the resulting reinforcing effect on the PEO matrix were studied. Polarizing optical microscopy (POM), scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric (TG) analysis, Fourier transform infrared spectroscopy (FTIR), and tensile testing were used to examine the properties of the composites. The revealed extreme dependences of the composite properties refer to the CNCs content of 15–35wt.%. This composition range is characterized by destruction of PEO spherulites and appearance of a pronounced PEO fibrillar structure.