Nylon 4/6 copolymers were synthesized by a melt-condensation method using E >-caprolactam and 2-pyrrolidone as monomers and 6-aminocaproic acid as a catalyst. Their structures and morphologies were studied using wide and small angle X-ray scatterings, differential scanning calorimetry, and scanning electron microscopy. The nylon 4 units were included into the nylon 6 chain up to 25 mol%. Random introduction of the small amount of the nylon 4 units to the nylon 6 decreased the crystallinities. The synthesized copolymers were successfully electrospun into nanofiber mats with diameter ranges of 90-130 nm, and their structures were compared to those of the films. The copolymers adopted the crystal structure of the pure nylon 6 with mixed nylon 4 and 6 units in the amorphous region, although the crystal structures of the nanofiber mats and the films were the gamma and alpha forms of nylon 6, respectively. The crystallinity of the nanofiber mats decreased compared to those of the films. The water absorption of the nylon 4/6 copolymers increased as the mol% of nylon 4 increased in the copolymers.