A hyperbranched poly[bis(hexaethylene glycol)benzoate] with terminal acetyl groups was prepared, and ionic conductivity, thermal property, and electrochemical stability of the polymer electrolyte with LiN(CF3SO2)(2) were investigated. Ionic conductivity of the polymer electrolyte exhibited a maximum value of 9 x 10(-5) S/cm at a [LiN(CF3SO2)(2)]/[repeat unit] ratio of 0.6 (corresponding to a Li/O ratio of 0.04) and 80 degreesC. The effect of a hyperbranched polymer, a poly[bis(triethylene glycol)benzoate] with terminal acetyl groups (poly-Ac1b), as an additive on ionic conductivity of a linear poly(ethylene oxide) (PEO)/LiN(CF3SO2)(2) electrolyte was also investigated. Addition of 10 wt.% poly-Ac1b improved ionic conductivity and also ion transference number in the polymer electrolyte.