This Letter reports the thermal, electrical and mechanical properties of the (PEO)(16)LiClO4 based composite polymer electrolytes with modified montmorillonites as fillers. As modified montmorillonites are added, the glass transition temperature and the heat of melting decrease, indicating the higher flexibility of PEO chain. Wide-angle X-Ray diffraction patterns indicate the intercalation of PEO into montmorillonite. The addition of modified montmorillonites enhances the ionic conductivity of the PEO based electrolyte by several times to tens more than the plain system. The mechanical properties of the polymer electrolyte films are related to the morphology of montmorillonites in the polymer matrix. (C) 2003 Elsevier Science B.V. All rights reserved.