This paper examines the impact of Cloisite15A filler on crystallinity, thermal transition (glass transition temperature, melting and crystallization temperature), thermal stability, structure of poly(ethylene oxide) (PEO) (molecular mass, ̅̅̅̅ =1 000 000) (PEO10) and ionic conductivity. Lithium bis(oxalato) borate (LiBOB) was added in a ratio of 1:8 (Li : oxygen of ether) to achieve a sufficient concentration of the charge carrier. Two series of polymer nanocomposites were prepared. The first one was obtained by mixing PEO10 with 15A filler at the appropriate ratios, using a melt intercalation method. Samples with mass ratios of PEO10/15A: 90/10, 80/20, 70/30, 60/40, 50/50, 40/60, 30/70, 20/80 and 10/90 were prepared. The second one was obtained by mixing PEO10 with 15A filler in the respective ratios with the addition of LiBOB by a melt intercalation method. Samples with mass ratios of PEO10/15A/LiBOB: 90/10, 80/20, 70/30, 60/40, 50/50, 40/60, 30/70, 20/80 and 10/90 were prepared. Differential scanning calorimetry (DSC) investigated the influence of 15A on the thermal transitions (glass transition, melting and crystallization) and crystallinity of PEO10. It was noted that the addition of 15A reduces the temperature of crystallization, but does not significantly affect PEO10 glass transition temperature and melting point temperature. Melting and crystallization entalphy reduces with the addition of 15A which suggests the reduction of the degree of crystallity of PEO10. Thermogravimetric analysis (TGA) investigated the influence of 15A on the thermal stability of PEO10. It was found that the starting temperatures of the decomposition (T o ) and the temperatures at the maximum decomposition rate (T max ) decrease with increasing mass. % of 15A. It was concluded that the addition of filler 15A reduces the thermal stability of PEO10. Fourier transform infrared spectroscopy (FT-IR) investigated the influence of 15A on the PEO10 structure. It was found that the addition of 15A influences the conformation of PEO10, that there is a distortion of its helical conformation. This indicated a change in crystallinity in PEO10 and is consistent with results obtained by differential scanning calorimetry (DSC). Electrochemical impedance spectroscopy (EIS) investigated the influence of 15A on ionic conductivity of PEO10. It was found that the sample with the addition of 10 wt. % shows maximum ionic conductivity, which suggest that the degree of crystallinity is not the only that affects the ionic conductivity of the nanocomposite.