Porous flat sheet membranes for separators in Zn air batteries were prepared with polyethersulfone (PES) solutionsby immersion precipitation phase inversion method. PES/polyvinylpyrrolidone(PVP)/N-methylpyrrolidone(NMP) mixtureswere used for casting solutions and water was used for coagulant. With the separators, Zn air batteries were fabricated.The separators were characterized by means of stress-strain test, impedance test and SEM. The Zn air batteries were testedby current interrupt method (CIM) and galvanostatic discharge method. The tensile strength increased with increasing PEScontent in the casting solution while the ionic conductivity decreased. On the other hand, the ionic conductivity increasedwhile the tensile strength decreased with increasing PVP content. The effect of ionic conductivity trend of the separator inthe Zn air battery was confirmed through current interrupt method and galvanostatic discharge method experiments. The batterywith the separator from casting solution with higher PES content showed higher IR drop and lower discharge capacity.And the battery with the separator from casting solution with higher PVP content showed lower IR drop and higher dischargecapacity.