A comparative study of the structure and morphology of nylon-12 and 10-polyurethane (10-PUR) lamellar crystals, was carried out. Lamellar crystals were obtained by isothermal crystallization from diluted solution. Electron diffraction of lamellae combined with WAXS data recorded from crystal sediments indicated that nylon-12 crystallized in either α-form or γ-form according to the solvent chosen for crystallization. The α-form was the crystal modification predominant in doubly oriented films of nylon-12 prepared by epitaxial crystallization. On the contrary, 10-PUR invariably crystallized in α-form regardless crystallization conditions. The α-form of nylon-12 and 10-PUR shared the same crystal structure with hydrogen-bonded sheets made of antiparallel chains and stacked with progressive shifting along both b and c directions. Lamellar crystals of nylon-12 in γ-form and 10-PUR in α-form displayed similar morphological features but only the former appeared to be sensitive to temperature. Upon heating the nylon-12 crystals near to melting, the real-time WAXS analysis evidenced the occurrence of a partial γ-to-α crystal transition, and in situ AFM observations revealed the appearing of more or less regular ridges on the crystal surface. None of these changes were observed in 10-PUR crystals when subjected to similar treatment.