Nylon 4/6 copolymers were synthesized by a melt-condensation method using ${varepsilon}$-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.