In this work, composite polymer electrolytes which consist of polyethylene oxide (PEO) as the polymer matrix, lithium tetrafluoroborate (LiBF4) as the lithium salt and functionalized SiO2 as the inert ceramic filler are prepared by a film-casting method. SiO2 is functionalized by 2-[methoxy(polyethylenoxy)-propyl] trimethoxy silane. X-ray photoelectron spectroscopy (XPS) and thermal gravimetric analysis (TGA) are used to characterize the functionalized SiO2. Compared with untreated SiO2 as the ceramic filler there is noticeable improvement in ionic conductivity, stability of the Li/polymer electrolyte interface, and the mechanical properties when functionalized SiO2 is used.