Poly(styrene sulfonic acid)-grafted poly(ethylene-cotetrafluoroethylene) (ETFE-PEM) was prepared using a pre-irradiation grafting method, in which polymer substrate (ETFE) was irradiated using gamma ray, then immersed into a monomer styrene solution for graft polymerization, and then we carried out the subsequent sulfonation on the graft polymer to obtain a proton exchange membrane. Lamellar structures of ETFEPEMs such as functions of grafting degree (GD) were investigated using small and ultra-small angle X-ray scattering. The obtained results showed that styrene was introduced into the amorphous phase of lamellar structures, resulting in the expansion of their thickness, and hence causing the increase in the lamellar period with GD = 0-34%. Crystalline lamellar thickness which relates to the mechanical integrity of the membranes decreased slightly when GD was ≥ 34%. Also, when GD was ≥ 34%, styrene did not come into the lamellar structures, but it was introduced into the lamellar grains. This process resulted in no change in the lamellar period of lamellar structures, whereas the lamellar grains developed significantly with GD ≥ 34%.