The ionic conductivity and electrode-electrolyte interfacial stability have been measured for composite polymer electrolyte using nano-size alumina (Al2O3) with poly-ethylene oxide (PEO) polymer, and lithium tetrafluoroborate (LiBF4). The ionic conductivity of ball-milled PEO electrolyte was increased by Al2O3-filler-addition (BCPE) about 10 times greater than that without Al2O3 (BPE) over a range of 30 degrees C to 80 degrees C. The stability of resistance between electrode and electrolyte was significantly improved by the addition of Al2O3 filler during storage time. Cyclic performance of Li/BCPE/Li cell was superior to that of Li/BPE/Li cell.