The polyamic acid (PAA), which is the precursor of polyimide (PI), is synthesized by pyromellitic dianhydride (PMDA) and 4, 4'-oxidianiline (ODA). The PAA/PET composite membrane is prepared with poly (ethylene terephthalate) (PET) nonwoven by electrospinning of the solution of PAA. The final PI/PET composite membrane is obtained after a thermal imidization process. The porosity, liquid electrolyte uptake and room temperature ionic conductivity of PI/PET composite membrane decreases a little comparing with PI membrane, but the tensile strength of PI/PET composite membrane increases more than PI ones. The experiments demonstrate that the diameter of the fibers decrease with the increase of the electrospinning voltage. At the same time, the decrease of the fibers diameter leads to the increase of the porosity as well as the liquid electrolyte uptake of the PI/PET composite membranes. As a result, the room temperature ionic conductivity is enhanced from 0.451 to 0.897 mS . cm(-1). The PI/PET composite membrane shows only about 2% of thermal shrinkage at 180 degrees C under air atmosphere. The cell with the PI/PET composite membrane shows stable cycling performances and better rate capabilities than the cells with the Celgard one under 10 C-rate. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.041209jes] All rights reserved.