Dielectric properties of proton conducting polymer electrolyte system, containing polyethylene oxide (PEO) as host polymer and ammonium hexfluorophosphate (NH4PF6) as complexing salt plasticized with EC and EC:PC, are investigated. The free standing films of thickness ~ 200 - 300µm are synthesized by solution casting technique. The electrical conductivity studies show that plasticization of the polymer electrolyte results into an enhancement in its conductivity by about two orders of magnitude. The maximum room temperature bulk conductivity is obtained to be ~ 10-5 S/cm for the plasticized polymer electrolyte. To understand the ion transport mechanism, different frequency dependent parameters are measured like dielectric permittivity, loss tangent and AC conductivity. The ionic transference number of the prepared systems is found to be close to unity which shows ion dominant charge transport in the electrolyte system. The conductivity of the polymer electrolyte has been found to be very sensitive to the relative humidity, which makes it a good candidate for its application for humidity sensor.