The lamellar thickness of poly(ethylene oxide) (PEO) film established from a gel state was investigated in an effort to understand the crystalline structure which forms from the cross-linked network. Crystalline films were prepared with PEO solutions irradiated by gamma-rays. The structural aspect of the films was revealed in terms of the exposed radiation energy and the polymer concentration by using differential scanning calorimetry, X-ray diffraction, and small-angle X-ray scattering. The long-period distance, lamellar thickness, and amorphous thickness in the crystalline films were analyzed based on the small-angle X-ray scattering patterns. The wide distribution and nonuniformity of the lamellar stacks were observed in the elongated and the swollen films. The lamellar thickness was compared to the chain lengths between the cross-links, and the possible presence of cross-links in the lamellar stacks was discussed. The results revealed the lamellar thickness influenced by the cross-links and demonstrated the structural changes associated with both the crystalline phase and cross-linking.