Novel polyvinyl formal (PVFM) based gel polymer electrolytes (GPEs) are developed using an initiator free thermal polymerization method. The polymerization mechanism during the cross-linking process is investigated by means of Fourier transform infrared (FTIR) spectroscopy measurements. With the prepared GPEs (containing 2 to 5 wt % PVFM), Li polymer batteries with LiFePO4 as the cathode are assembled, and the electrochemical properties such as interfacial impedance, electrochemical stability window and cycling performance are evaluated. The resulting PVFM based GPEs present a better thermal stability compared with the corresponding conventional liquid electrolyte and an acceptable conductivity of ∼10−3 S cm−1 at ambient temperatures. Cyclic voltammetric (CV) curves reveal that the electrochemical stability window of PVFM based GPE is 1.5–5V vs. Li/Li + and wider than that for the corresponding liquid electrolyte which is 1.8–4.4 V. The discharge capacity of the polymer Li/LiFePO4 battery is 145 mAh g−1 over a voltage range of 2.5–4.25 V at 1/10 C rate after 80 cycles with a small capacity fade.