In this present work, a polymer electrolyte based on polymer/clay nanocomposite has been developed. Montmorillonite (MMT) clay was used as the filler, due to its special size in length and thickness, and its sandwich type structure. The obtained gel polymer electrolytes have high ionic conductivity up to 2.5mScm−1 with high cationic transference number (about 0.64) at room temperature. The influences of the filler on the membrane morphology, the solvent uptake, the ionic conductivity, and the cation transport number were investigated, and thus the significant contribution from the exfoliated organophilic MMT was identified.