Solid polymer electrolytes consisting of CO2-derived poly(ethylene carbonate) (PEC), LiPF6, and plasticizers (glycerol or 1-ethyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide, EMImTFSI) were prepared by a simple casting method, and their dielectric relaxation behavior was evaluated using broadband electric spectroscopy (BES), which clarified the correlation between the polymer motion and ionic conduction. From the DSC and BES results, it was revealed that the addition of plasticizer decreased the glass transition temperature and increased the dc conductivity (σdc) of the PEC electrolyte. The BES results also revealed that the plasticizer increased the segmental motion of PEC and improved σdc, and the plasticizing effect of EMImTFSI on the PEC electrolyte was larger than that of glycerol. From the results of the Walden plot and fragility analysis, it was expected that the degree of decoupling ε and fragility m would increase with the addition of plasticizer because these plasticizers weaken the interactions between the PEC chains and Li ions in the electrolyte.Plasticized poly(ethylene carbonate) (PEC)/LiPF6 electrolytes were prepared and evaluated their ion-conductive and dielectric relaxation behavior using broadband electric spectroscopy (BES). The BES results indicated that the plasticizer accelerates segmental motion of PEC and improve the dc conductivity, and the plasticizing effect of ionic liquid (EMImTFSI) on the PEC electrolyte is larger than that of glycerol. From the results of the Walden plot and fragility analysis, it was revealed that the degree of decoupling and the value of fragility increase by the addition of plasticizer, and these plasticizers weaken interactions between PEC chains and Li ions in the electrolyte.