The hierarchical structures of graft-type poly-(ethylene-co-tetrafluoroethylene) (ETFE)-based polymer electrolyte membranes (ETFE-PEMs) were investigated using small- and ultrasmall-angle X-ray scattering experiments. The ETFE-PEMs with ion exchange capacities (IECs) <2.4 mmol/g possessed conducting graft domains around lamellar crystals, with a d-spacing of 21.8-29.1 nm, and oriented crystallites (lamellar grains) with short and long correlation distances of 218-320 and 903-1124 nm, respectively. The membranes with IECs > 2.7 mmol/g showed a new phase of crystallite network domains with a d-range of 225-256 nm, indicating a phase transition from oriented crystallite to crystallite network structures in the IEC range of 2.4-2.7 mmol/g. Noted that for the ETFE-PEMs with high IECs higher conductivity at 30% RH and compatible tensile strengths at 100% RH and 80 degrees C, compared with Nafion, originated from the well-interconnected ion channels around the crystallites and the remaining lamellar crystals and crystallites, respectively.