In this contribution, a solid polymer electrolyte (SPE) was prepared by a ball milling method followed by a hot pressing process. The SPE used poly(ethylene oxide) (PEO) doped with trifluoromethanesulfonate (LiCF3SO3) as a host matrix. Nano alumina (Al2O3) was used as filler, and dioxyphthalate (DOP) and polyethylene glycol (PEG) applied as plasticizers. Various concentrations of lithium salts, nano alumina and plasticizers (5, 10, 15 and 20wt%) were studied. The morphology, ionic conductivity, thermal and mechanical properties of the SPE were also examined. The ionic conductivity increased from 10−7 to 10−6 Scm−1 upon the addition of lithium salt. The PEO-15wt% (LiCF3SO3) solid polymer electrolyte exhibited maximum room temperature ionic conductivity. Significantly, the addition of nano alumina and plasticizers into PEO-LiCF3SO3 SPE enhanced the ionic conductivity to 10−5 and 10−4 Scm−1, respectively. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) studies indicated that the conductivity increase was due to the decrease of crystallinity upon the addition of lithium salt, nano alumina and plasticizers into the SPE system. The mechanical properties of PEO tended to decrease with the addition of lithium salts, nano alumina and plasticizers. Nevertheless, durable film with high flexibility could be formed with a potential use as an efficient electrolyte in lithium and other rechargeable batteries.