A separator for metal/halogen batteries is provided which is excellent in thermal resistance, resistance to stress-cracking and electric properties. A separator for metal/halogen batteries comprising a polyolefin having a viscosity average molecular weight of not less than 350,000 and not more than 2,000,000, in which polyolefin the proportion by weight of a polyolefin having a molecular weight of not less than 1,000,000 is not less than 15% and not more than 80%, and a hydrophilic fine particulate inorganic material(s), wherein the weight ratio of the polyolefins to the hydrophilic fine particulate inorganic material(s) is not less than 0.55 and less than 1, said separator having a pore volume of not less than 900 mm.sup.3/g and a thickness of 0.2 to 1.0 mm.