The ionic liquid of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) is selected as plasticizer to prepare a series of novel gel polymer electrolytes due to its non-volatility and inflammability. The micro-porous matrix employed in this study is poly(vinylidene fluoride-co-hexafluoropropylene)/poly(methyl methacrylate) (PVDF-HFP/PMMA) membrane synthesized via phase inversion. The effect of the ratio of PVDF-HFP to PMMA on the properties of the micro-porous membrane, such as morphology, porosity and the liquid uptake capability are systematically studied. Thermal and electrochemical properties of the resulting gel polymer electrolyte are also studied with the objective to optimize the quality of the micro-porous gel polymer electrolyte. The results indicate that the micro-porous gel polymer electrolyte MIL-60, derived from the matrix of PMMA-60 (40 wt% PVDF-HFP and 60 wt% of PMMA), presents the lowest glass transition temperature and exhibits the highest ionic conductivity. The LiFePO4/MIL-60/Li coin-typed cell cycled at 0.1C shows the 1st discharge capacity up to 157 mAh·g−1, and remains 151 mAh·g−1 on the 50th cycle.