Cross-linked polymer electrolytes are prepared from oligo(ethylene glycol) dimethacrylates (OEGDMA) with different ethylene oxide repeating units in the presence of lithium perchlorate as a lithium salt, ethylene carbonate–propylene carbonate as a mixed plasticizer, and poly(ethylene oxide) as a polymer matrix. The ionic conductivities of polymer electrolytes increase with an increasing amount of the ethylene oxide repeating unit in the OEGDMA. Conversely, the glass transition temperatures of the polymer electrolytes decrease with an increasing amount of the unit. The increased ionic conductivity can be explained by the sites of the spaces surrounded by the main chains and the cross-linking spacers in the polymers.