Polyimide (PI) / Polyethylene terephthalate (PET) composite membranes are prepared by the phase inversion process from casting solutions composed of PI, N-methyl-2-pyrrolidone (NMP) and PEG. The porosity, liquid electrolyte uptake and room temperature ionic conductivity are investigated. The experiments demonstrate that the average pore size of the membranes increase with increasing the weight ratio of PEG. At the same time, it leads to the increase of the porosity, liquid electrolyte uptake and room ionic conductivity. The PI microporous membrane shows only about 4% of thermal shrinkage in at 180 degrees C under air atmosphere. The cell with the PI/PET composite membrane shows stable cycling performances through the charge-discharge tests with capacity retention ratios of 86.7% covering 50 cycles. The cell with the PI/PET composite membrane shows better rate capabilities than the cell with Celgard one under 10 C-rate. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.023208jes] All rights reserved.