Electrochemical and interfacial properties of (PEO)10LiCF3SO3-Al2O3 composite polymer electrolytes (CPEs) prepared by either ball milling or stirring are reported. Ball milling was introduced into a slurry preparative technique utilizing PEO, lithium salt and Al2O3 powder ranging from 5 to 15 wt.%. The ionic conductivity was increased by ball milling over a range of temperatures. In particular, a significant increase at low temperature below the melting point of crystalline PEO was observed. Interfacial stability between lithium electrode and CPE was significantly improved by the addition of alumina as well as by ball milling. The electrochemical stability window produced by (PEO)10LiCF3SO3-Al2O3 ball milling was higher than that of stirring, which was about 4.4 V. Charge/discharge performance of Li/CPE/S cells with (PEO)10LiCF3SO3-Al2O3-12 hr ball milling was superior to that of a pristine polymer electrolyte due to the low interface resistance and high ionic conductivity.