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 9×10−5 S/cm at a [LiN(CF3SO2)2]/[repeat unit] ratio of 0.6 (corresponding to a Li/O ratio of 0.04) and 80°C. 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 10wt.% poly-Ac1b improved ionic conductivity and also ion transference number in the polymer electrolyte.