Nano-sized magnesium-benzene dicarboxylic acid metal-organic framework (Mg-BDC MOF) was successfully synthesized via anodic dissolution of Mg2+ ions into the reaction medium by using the electrochemical method (EC) without metal salts. Nano-sized Mg-BDC MOF structures with high purity and high yields (similar to 81%) were obtained and used as a new nanofiller in Poly (ethylene oxide) PEO-based polymer matrix. The crystalline structure of nano-sized Mg-BDC MOF were confirmed by the characteristic diffraction peaks based on X-ray diffraction (XRD) analysis. The spherical shape and smooth structure of nano-sized Mg-BDC MOF were determined by scanning electron microscopy (SEM) and atomic force microscopy (AFM) analyses. Their dimensions were found to be in the range of similar to 20 nm. PEO nanocomposite electrolyte consisting of Mg-BDC MOF was successfully prepared by using solution casting method. The homogeneous distribution of nano-sized Mg-BDC MOF nanoparticles within the PEO electrolyte matrix was examined by SEM analysis. The reason for the size reduction and uniform distribution of nano-sized Mg-BDC MOF was expressed as the positive physical interactions between the surface functional groups of PEO and Mg-BDC during the solution casting process and the absence of any possible Mg-BDC nanoparticle aggregation.