Polymer electrolytes composed of poly(ethylene oxide) and a polyanionic lithium salt are prepared, and electrochemical and thermal properties were investigated in the absence and presence of BF3·OEt2. The addition of BF3·OEt2 increases the ionic conductivities, and the lithium ion transference number of the polymer electrolyte for the PEO/poly(lithium acrylate) (PLA) and PEO/poly(lithium vinylsulfonate) (PLVS) system were higher than that of PEO/lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) system. However, ionic conductivities of the polymer electrolyte in the presence of BF3·OEt2 for the PEO/PLVS system were higher than that for the PEO/PLA system below 50°C, due to the suppression of the crystallization of PEO. It was found that the structure of functional group in polyanionic lithium salts significantly affect electrochemical and thermal properties of polymer electrolytes.