Solid polymer electrolytes (SPE) are expected to be useful as an electrolyte for batteries because of its safety, flexibility and lightweight. Many attempts for polyether type SPE have since been made to improve their electrochemical and physicochemical properties. Poly(ethylene oxide) (PEO) and its derivatives have usually low glass transition temperatures (T g) and those electrolytes exhibit relatively high ionic conductivity. However, PEO-based electrolytes suffer from low Li transference numbers (t +), less than 0.3. In PEO-based electrolytes, a cross-linking structure based on strong ion-dipole interactions between cations and polar ether oxygen is usually formed. Ionic conduction in PEO-based electrolytes therefore strongly depends on the migration of anions, which is known to occur in materials unsuitable for practical batteries. To overcome these problems we recently suggested polycarbonates, such as poly(ethylene carbonate) (PEC), as a new polymer matrix for SPE. However, PEC-based electrolytes have a issue with low ionic conductivity relative to PEO-based electrolytes at low salt concentrations, because of the high T g of the original polymer. In this study, we focus on the copolymerization of ether and carbonate units as novel candidates for the electrolytes. The ether/carbonate copolymers can be obtained with suitable ratio by the optimization of the polymerization condition. We investigated the influence of the ether units on the ion-conductive properties of poy(ethylene oxide/ethylene carbonate) (P(EO/EC))-based electrolytes. The electrolytes were prepared with lithium bis(fluorosulfonyl)imide (LiFSI). The LiFSI content in the electrolyte varied from 5 to 160 mol% to an average monomer unit of each EO and EC unit. We found that the T g of neat copolymers and the electrolytes decreased with increasing the contents of EO unit, while the conductivity for the electrolytes increased with increasing both EO unit and salt concentration. The t + of P(EO/EC) electrolyte having EO unit of 53% and 80mol% of LiFSI had high t + 0.65 at 60 oC. This value was very high and comparable with that of the PEC-based electrolytes. The dissociation states of Li ions is essential to reveal ion-conductive properties of P(EO/EC) electrolytes, so the measurement of FT-IR was carried out. The results of characterization of FT-IR spectra suggest that Li ions interact preferentially with carbonate groups but hardly with ether groups in P(EO/EC). Thus, the P(EO/EC) electrolytes showed novel ion-conductive behavior which has advantages of both EO and EC-based electrolytes. The optimization of the copolymerization ratio for EO and EC units in the copolymers are expected to further improvement in the conductivity. This work was supported financially by a Grant-in-Aid for Scientific Research (B) (No. 25288095, 16H04199) of JSPS KAKENHI ,Japan.