The influence of the hyperbranched polymer (HBP) structure such as molecular weights, molecular weight distribution, chain-end, ethylene oxide (EO) chain lengths on the ionic conductivity of the composite polymer electrolytes composed of poly(ethylene oxide) (PEO), BaTiO3 as a ceramic filler, LiN(CF3SO2)(2) as a lithium salt, and HBP as a plasticizer were investigated. The difference in the molecular weights of the HBP did not affect significantly the ionic conductivity. However, molecular weight distribution of the HBP might affect the ionic conductivity of the composite polymer electrolyte, which decreased with broadening of the molecular weight distribution. Further branching at the chain-end structure in the HBP led to a decrease in the ionic conductivity. The HBP with a longer EO chain length was effective to an enhancement of the ionic conductivity. (C) 2004 Elsevier Ltd. All rights reserved.