The variation of lamellar structures of poly(styrenesulfonic acid)-grafted poly (ethylene-co-tetrafluoroethylene) proton exchange membranes dependence on preparation procedures and grafting degree (GD) was investigated by small angle X-ray scattering. The detail structures of lamellar including lamellar period L, thickness of lamellar crystal Lc, thickness of lamellar amorphous La, and linear crystallinity Lc/L were examined by a 1D correlation function. The lamellar structures were recognized at the grafting step and did not change under the sulfonation process. With GD  79 %, Lc significantly decreased (corresponding to the increase of La) and then retained in the GDs of 79-117 %. Note that the retained values of Lc, La, and linear crystallinity in the GDs of 79-117 % are the origin of high conductivity and mechanical strength of membranes under severe operation conditions for fuel cell applications.