We investigated the effects of PEG–borate ester on the ionic conductivity and transport number of lithium ions in polymer electrolytes with Li-salt, such as LiN(CF3SO2)2, LiClO4 or LiCF3SO3. The ionic conductivity of the polymer electrolyte with LiN(CF3SO2)2 was found to be higher than those of the electrolytes with LiClO4 or LiCF3SO3. This result is mainly concerned with mobility of ions, which is indicated by the comparison of glass transition temperatures of the polymer electrolytes. On the other hand, the transport numbers of lithium ions in the polymer electrolytes with LiClO4 or LiCF3SO3 were higher than those of the electrolyte with LiN(CF3SO3)2. Furthermore, the conductivities of lithium ions calculated by multiplying the ionic conductivity measured by ac impedance with transport number of lithium ions were also found to be higher than that of the electrolytes with LiN(CF3SO2). These results indicate that the PEG–borate ester acts as Lewis acid and interacts with CF3SO3 − or ClO4 − more effectively compared with N(CF3SO2)2 −, leading to improved dissociation of Li-salts and transport number of lithium ions.