In this article, we report a novel gel polymer electrolyte (GPE) for lithium ion batteries, which is prepared using poly(methyl methacrylate-acrylonitrile-ethyl acrylate) (P(MMA-AN-EA)) as a polymer matrix and doping with nano-SiO₂ and nano-Al₂O₃ simultaneously. The influences of the ratio of the two nanoparticles on the pore structure, electrolyte uptake and thermal stability of the resulting membrane, and the ionic conductivity and electrochemical stability of the corresponding GPE are investigated by scanning electron microscopy, mechanical strength, thermogravimetry, electrochemical impedance spectroscopy, linear sweep voltammetry and cyclic voltammetry. The performance of the developed GPE is evaluated in the Li/LiNi_0.5Mn_1.5O₄ half cell by a charge–discharge test for its application in lithium ion batteries. It is found that there exists a synergistic effect between nano-SiO₂ and nano-Al₂O₃. The performances of the resulting membrane and the corresponding GPE are effectively improved by using nano-SiO₂ and nano-Al₂O₃ simultaneously rather than individually. Co-doping 5 wt% nano-SiO₂ and 5 wt% nano-Al₂O₃ provides the membrane with a higher thermal decomposition temperature of 325 °C, and a better electrolyte uptake of 198.1%, the corresponding GPE with an increased ionic conductivity of 2.2 × 10⁻³ S cm⁻¹ at room temperature and an enhanced oxidative stability up to 5.5 V (vs. Li/Li⁺), and the LiNi_0.5Mn_1.5O₄ cathode with an improved rate capability of 104.2 mA h g⁻¹ at 2C and an improved capacity retention of 94.8% after 100 cycles. These improved performances result from combining the advantages of both nano-SiO₂ and nano-Al₂O₃, in which the former contributes to the improved ionic conductivity caused by a stronger Lewis-acid property, while the latter to the better thermal and structural stabilities by its stiffness characteristic.