In our previous study, we synthesized and characterized nano-sized metal organic framework-5 (MOF-5) by using consecutive combination of ultrasound and microwave irradiation methods. In the current study, we synthesized PEO nanocomposite electrolytes consisting of these nano-sized MOF-5 structures by using the solution casting method. The molecular interactions, crystallinity, thermomechanical properties and thermal stability of poly(ethylene oxide) nanocomposite electrolytes were investigated by Fourier-transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA). In addition, the morphology, crystalline structure and ionic conductivity of PEO nanocomposite electrolytes were analyzed by using scanning transmission electron microscopy (STEM), X-ray powder diffraction (XRD) and AC impedance spectroscopy, respectively. Based on the detailed analysis and comparison of this data, PEO nanocomposite electrolytes show considerable property improvements such as higher modulus and much lower crystallinity and comparable thermal stability compared to pure PEO electrolyte. Since, all of these properties are very important in the performance of solid polymer electrolytes for rechargeable polymer lithium battery applications, PEO nanocomposite electrolytes containing nano-sized MOF-5 have a high potential to be used as solid electrolyte membranes in battery applications. Keywords: PEO electrolytes, nanocomposite, metal organic framework, rechargeable polymer lithium battery