Composite polymer electrolyte films consisting of poly(acrylonitrile-co-methyl methacrylate) (P(AN-co-MMA)), EC-PC, LiClO4 salts, and also silica particles have been prepared, and their electrochemical properties were studied. A maximum in the ion conductivity was found for the plasticized polymer electrolyte containing 10 wt% silica with EC-PC and was 1.7 x 10(-4) S/cm at - 15 degreesC and 1.9 x 10(-3) S/cm at 25 degreesC. The reduction in T-g of the plasticizer-rich phase in the composite polymer electrolytes due to the interaction between the added silica and lithium cation seems to be a main reason for the initial increase of the ion conductivities with the increase in the silica content. The decrease in the ion conductivity with the silica content at the higher silica content is expected to be attributed to the increase of viscosity of the plasticizer-rich phase and the decrease in the charge carrier numbers. The stability of interfacial resistance between the composite polymer electrolyte and the lithium electrode was found to be enhanced with increase of the silica content in the range studied.