Membranes, which consist crosslinked polymer electrolyte, modified structures by using poly(styrene-b-butadiene-b-styrene) (SBS) triblock copolymer and a sulfonated monomer, 2-sulfoethyl methacrylate (SEMA). With thermal-initiator, 4,4'-azobis(4-cyanovaleric acid), the SEMA membranes were crosslinked. Owing to increase of SO3-groups, the water uptake and ion exchange capacity of membranes was enhanced. By using the series of the equipment, like wide angle X-ray scattering, small angle X-ray scattering, and transmission electron microscopy, microphase-separated morphology and amorphous structures of crosslinked SBS/SEMA membranes was identified. Universal testing machine and thermo gravimetric analysis were performed to investigate mechanical properties and thermo behavior of synthesized polymer membrane.