The effect of plasticizer (polyethylene glycol, PEG(200)) on dielectric and electrical properties of an ionically conducting polymer nanocomposites PEO25-NaI + 5 wt.% DMMT (dodecyl amine modified Na+-montmorillonite)+x wt.% PEG200 with different value of x has been investigated. At low frequency the, variation of relative dielectric constant with frequency shows the presence of material electrode interface polarization processes. The tangent loss peaks appearing at a characteristic frequency suggest the presence of relaxing dipoles in all the samples. On addition of plasticizer, the peak shifts towards higher frequency side suggesting the speed up the relaxation time. The electrical modulus formalism is used to study the ionic relaxation processes in these composites in terms of conductivity relaxation time. Analysis of electrical modulus and dielectric permittivity functions suggest that ionic and polymer segmental motions are strongly coupled. The frequency dependence of ac conductivity follows the universal power law with a small deviation in the low frequency region due to the electrode polarization effect. (C) 2009 Elsevier B.V. All rights reserved.