Measurements of small-angle x-ray diffraction (SAXD) were performed for nylon 6 samples quenched from the melt at various crystallization temperatures T (°K). The spacing estimated from the position of maximum intensity in a SAXD diagram, which is assumed to be equal to the thickness of lamellar crystals ℓ, decreases with the decrease of T. The behavior was analyzed in terms of nucleation theory. In applying the theory. the contribution of γ-form crystals for the samples crystallized at lower T was considered. For this purpose wide-angle x-ray diffraction and infrared data obtained for the same samples used in SAXD measurements were used. It was shown that the relation of ℓ versus T deviates remarkably from the theoretical prediction under the premise that the surface free energy σe(ergs/square centimeter) is constant with T. We present an approach to the analysis of the data from the standpoint that σeis a parameter changing with T, where σeshould be interpreted as related to the degree of irregularity of the fold surface of lamellar crystals. The values of σethus obtained increase with increasing undercooling δT, where δT = Tm− T, with Tm(°K) being the melting point. SAXD data of polyethylene reported earlier by Mandelkern et al. were analyzed from the same standpoint, and it was found that the values of σeincrease with δT/Tmin a similar manner for both nylon 6 and polyethylene.