Polyvinyl formal (PVFM)-based dense polymer membranes with nano-Al2O3 doping are prepared via phase inversion method. The membranes and also their performances as gel polymer electrolytes (GPEs) for lithium ion battery are studied by field emission scanning electron microscope, X-ray diffraction, differential scanning calorimetry, mechanical strength test, electrolyte uptake test, electrochemical impedance spectroscopy, cyclic voltammetry, and charge-discharge test. The polymer membrane with 3 wt % nano-Al2O3 doping shows the improved mechanical strength of 12.16 MPa and electrolyte uptake of 431.25% compared with 10.47 MPa and 310.59% of the undoped sample, respectively. The membrane absorbs and swells liquid electrolyte to form stable GPE with ionic conductivity of 4.92 x 10(-4) S cm(-1) at room temperature, which is higher than 1.77 x 10(-4) S cm(-1) of GPE from the undoped membrane. Moreover, the Al2O3-modified membrane supporting GPE exhibits wide electrochemical stability window of 1.2-4.8 V (vs. Li/Li+) and good compatibility with LiFePO4 electrode, which implies Al2O3-modified PVFM-based GPE to be a promising candidate for lithium ion batteries. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2014, 52, 572-577