The interfacial characteristics between the lithium (Li) electrode and the plasticized polymer electrolyte composed of poly(acrylonitrile-co-methyl methacrylate) [P(AN-co-MMA)], ethylene carbonate (EC), and LiPF6 salt was investigated by means of a.c. impedance spectroscopy. The Li electrode/P(AN-co-MMA)-based plasticized polymer electrolyte/Li electrode symmetric cell showed relatively stable interfacial resistance under prolonged storage time. This behavior is due to the enhanced compatibility between the matrix polymer and the organic solvent, EC, in the plasticized polymer electrolyte, which could lead to a suppression of leakage of the organic solvent from the polymer electrolyte film into the Li electrode.