Nylons have great potential for electrical applications requiring high polarizability and low dielectric loss. Recently, the narrow single hysteresis loop with relaxor ferroelectricity and the double hysteresis loop due to antiferroelectricity have been reported in nylon random copolymers, terpolymers, and common even-numbered nylons. Although several studies of ferroelectric nylons have been reported, even–even-numbered and odd–even-numbered nylons have not been sufficiently explored. Here, the ferroelectricity of spin-coated even–even-numbered and odd–even-numbered nylons was investigated. A series of even–even-numbered nylons, including nylons 6-12, 10-12, and 12-12, and an odd–even-numbered nylon, nylon 11-12, were polymerized with 1,10-dodecanedicarboxylic acid (12) and four aliphatic diamines with various methylene units, 1,6-hexanediamine (6), 1,10-decanediamine (10), 1,11-undecanediamine (11), and 1,12-dodecanediamine (12). The obtained nylon polymers were spin coated and then subjected to melt-quenching or thermal annealing followed by quenching. From the X-ray diffraction and the electrical hysteresis loop data, the correlation between the ferroelectricity and the crystal parameters of crystallinity and crystallite size of the γ crystal phase was investigated. Furthermore, the free volume of the nylon samples was estimated to correlate with the ferroelectricity. Temperature-dependent ferroelectricity was investigated for nylon 10-12. At a high temperature, the nylon samples showed a narrow polarization–electric field hysteresis loop and a rhombus-shaped polarization current–electric field hysteresis loop due to the relaxor ferroelectricity. This behaviour was caused by electrically rotating the nanodomains with weakened hydrogen bonds at higher temperatures.