The structural changes in the Brill transition of aliphatic Nylons m/ n have been investigated by carrying out the temperature-dependent infrared spectral measurement for Nylon 6/10 –[–NH–(CH 2) 6–NHCO–(CH 2) 8–CO–] n –, Nylon 6/12 –[–NH–(CH 2) 6–NHCO–(CH 2) 10–CO–] n – and Nylon 10/10 –[–NH–(CH 2) 10–NHCO–(CH 2) 8–CO–] n –. They took the crystal structure of α-form at room temperature. By heating the samples up to the Brill transition temperature region, the conformation of the methylene segments was found to change remarkably from the all trans-zigzag form to the disordered conformation constructed by shorter trans-zigzag segments combined with some gauche bonds. At the same time, the twisting motion was found to occur around the CH 2–amide bonds. During this order-to-disorder transition of molecular conformation, the intermolecular hydrogen bonds were kept alive although they were weaker gradually with increasing temperature. The methylene segments sandwiched by NH groups [NH(CH 2) m NH] were found to be disordered more remarkably than those of CO(CH 2) n CO sequence due to the difference in torsional energy barriers around the CH 2–NH and CH 2–CO bonds.