We prepared N-methyl-substituted polyurethanes with different substitution degrees from sodium hydride, methyl p-toluene sulfonate, and polyether-polyurethane containing poly(oxytetramethylene) glycol, 4,4'-diphenylmethane diisocyanate, and 1,4-butanediol. The chemical structures were characterized with Fourier transform infrared and H-1 NMR. To investigate the effects of the N-substitution degree on the morphology, thermal stability, and mechanical properties, we used differential scanning calorimetry, thermogravimetric analysis, and a universal testing machine. As the substitution degree increased, the new free (1708 cm(-1)) and bonded (1650 cm(-1)) carbonyl peaks increased. There was no bonded carbonyl peak in fully substituted polyurethane because the urethane groups had no hydrogen. At a small substitution degree, we observed a slight increase in the glass-transition temperature and decrease in the endotherms of soft-segment and hard-segment domains due to the decrease in the hard-segment domain and the increase in the urethane groups in the soft-segment domain. The hard-segment domain decreased and then disappeared as the N-methyl substitution degree increased. These changes in the morphology resulted (1) in decreased modulus and tensile strength for the films because of the decrease in physical crosslinking points and (2) improved thermal stability as the substitution degree increased.