Composite polymer electrolytes composed of a hyperbranched poly[bis(triethylene glycol)benzoate] with terminal acetyl groups, LiN(CF3SO2)2 as a lithium salt, and an inert ceramic filler such as α-LiAlO2 or γ-LiAlO2 were prepared by solvent casting method. Addition of an appropriate amount of the fillers to fully amorphous pristine polymer electrolytes led to an increase in ionic conductivities and lithium ion transference numbers. All composite polymer electrolytes exhibited good compatibility with a lithium metal electrode, and also, addition of fillers improved their mechanical performance. The α-LiAlO2 filler was effective for improving the electrochemical compatibility with a lithium metal electrode, and the γ-LiAlO2 filler was effective for enhancing the mechanical properties of the pristine polymer electrolytes.