Gel nanocomposite polymer electrolyte (NCPE) was prepared by UV polymerization and thermal polymerization, respectively in the presence of liquid electrolyte with nanosize SiO2-contained poly(ethylene glycol) diacrylate (PEGDA) as the monomer. Nanosize SiO2-contained PEGDA was synthesized using aqueous colloidal silica as one of starting materials and its viscosity was very low. The partial silanol surface groups of SiO2 were modified to acrylic group by employing of methacryloxypropyl-trimethoxysilane (MAPTMS), which made the dispersion of nanosize SiO2 in PEGDA uniform and stable. Compared with the gel polymer electrolyte (GPE) based on PEGDA without nanosize SiO2, the ionic conductivity of the gel NCPE was higher and the electrochemical stability and interfacial stability were better, whether it was prepared by UV polymerization or thermal polymerization. It showed oxidation stability up to 5.0V versus Li/Li+ and lithium deposition/dissolution on the stainless steel electrode highly reversible. The applicability of the gel NCPE to lithium polymer batteries was demonstrated by graphite/SPE/LiCoO2 cell, which was prepared by in situ thermal polymerization. It showed that the discharge capacity was stable with charge–discharge cycling.