In this work, 1–butyl–3–methyl imidazolium trifluoromethanesulfonate (BMITf) is employed as ionic liquid in polymer blend electrolyte system based on poly(ethyl methacrylate) (PEMA) and poly(vinylidenefluoride–co–hexafluoropropylene) (PVdF–HFP) in the weight ratio of 70 to 30. The films were prepared by solution casting. Lithium trifluoromethanesulfonate (LiTf) is the lithium ion (Li+) provider. Addition of BMITf into the optimized PEMA/PVdF–HFP–LiTf composition increased the ionic conductivity of the system. The highest room temperature ionic conductivity of 8.59×10−5 S cm−1 was exhibited by PEMA/PVdF–HFP–LiTf containing 60wt.% BMITf. Vibrational studies by FTIR show that the imidazolium cation (BMI+) interacts with the oxygen atoms in C=O and C–O–C groups of PEMA, and also fluorine atoms in CF2 groups of PVdF–HFP. Addition of BMITf into the PEMA/PVdF–HFP–LiTf system increases the amount of free ions leading to improvement in the ionic conductivity. The thermal stability of the polymer electrolytes is improved to above 275°C by the addition of BMITf.