Solid polymer electrolytes (SPEs) rapidly attracted a great attention as a potential material to be used in flexible rechargeable lithium ion batteries, if an optimal match between their ionic conductivity and mechanical stability was achieved. In this work, two types of nanocomposite SPE based on poly(ethylene) oxide (PEO) and lithium perchlorate salt (LiClO4) containing pristine graphene (GnP) or polyethylene glycol-grafted graphene (FGnP) were prepared. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) results revealed a reduced degree of crystallinity for nanocomposite electrolytes. The increment in amorphous phase domains was also confirmed by polarized optical microscope (POM), particularly for SPE/FGnP nanocomposites. Ionic conductivity showed one order of magnitude enhancement for SPE/FGnP(0.5%) at room temperature, while mechanical stability was also improved reasonably. Polyethylene glycol grafted onto FGnP appears to play two roles: plasticizing effect to promote the segmental motion of PEO chains, and contributing in dissociation of lithium salt. (C) 2017 Elsevier B.V. All rights reserved.