The interface resistance between Li x FePO4 and poly(ethylene oxide) (PEO)–Li(CF3SO2)2N (LiTFSI) was examined by AC impedance measurement of a Li x FePO4/PEO–LiTFSI/Li x FePO4 cell in the temperature range of 30–60°C. Four types of resistance, R0, R1, R2 and R3 were proposed according to analysis of the cell impedance using an equivalent circuit. The sum of R0 and R1 in the high frequency range is consistent with the resistance of the PEO electrolyte. R2 in the middle frequency range is related to lithium ion transport to an active point for charge transfer inside the composite electrode, and R3 in the low frequency range is considered to be the charge transfer resistance. The activation energy for R2 was affected by the thickness and composition of the electrode, whereas that for R3 was not.