Solid polymer electrolytes consisted of poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA) blend (50:50 wt/wt%) with lithium triflate (LiCF3SO3) as a dopant ionic salt at stoichiometric ratio [EO + (C=O)]:Li+ = 9:1, poly(ethylene glycol) (PEG) as plasticizer (10 wt%) and montmorillonite (MMT) clay as nanofiller (3 wt%) have been prepared by solution cast followed by melt-pressing method. The X-ray diffraction study infers that the (PEO-PMMA)-LiCF3SO3 electrolyte is predominantly amorphous, but (PEO-PMMA)-LiCF3SO3-10 wt% PEG electrolyte has some PEO crystalline cluster, whereas (PEO-PMMA)-LiCF3SO3-10 wt% PEG-3 wt% MMT electrolyte is an amorphous with intercalated and exfoliated MMT structures. The complex dielectric function, ac electrical conductivity, electric modulus and impedance spectra of these electrolytes have been investigated over the frequency range 20 Hz to 1 MHz. These spectra have been analysed in terms of the contribution of electrode polarization phenomenon in the low frequency region and the dynamics of cations coordinated polymer chain segments in the high frequency region, and also their variation on the addition of PEG and MMT in the electrolytes. The temperature dependent dc ionic conductivity, dielectric relaxation time and dielectric strength of the plasticized nanocomposite electrolyte obey the Arrhenius behaviour. The mechanism of ions transportation and the dependence of ionic conductivity on the segmental motion of polymer chain, dielectric strength, and amorphicity of these electrolytes have been explored. The room temperature ionic conductivity values of the electrolytes are found similar to 10 (5) S cm (1), confirming their use in preparation of all-solid- state ion conducting devices. (C) 2014 Elsevier B.V. All rights reserved.