A mixture of a vinyl monomer and a crosslinking agent was photopolymerized to form a crosslinked polymer film. Methacrylate with pendant oligo(oxyethylene) chain and poly(ethylene glycol) dimethacrylate were used as the vinyl monomer and crosslinking agent, respectively. The ionic conductivity of the film increased with an increasing concentration of LiClO4, and then decreased. The size of the quasicrystalline aggregation phase composed of pendant and crosslinking chains in the film decreased with an increasing concentration of LiClO4. The amorphous pendant and crosslinking oxyethylene chains gave rise to increased segmental motion and conductivity. The dissolution was depressed for a decrease in the ionic conductivity at a high LiClO4, concentration at which the interactions among ions became stronger and the crystal phase of LiClO4, was formed. The amount of the dissolution of the aggregation phase increased with an increasing crosslinking agent concentration. The quasicrystalline aggregation phase became larger with an increasing length of the pendant chains, and an increase in the size of the aggregation phase resulted in a decrease in the ionic conductivity.