Determining the thermal history of a semicrystalline polymer is an area of extreme importance when it comes to understanding the structure-propety relationships. For nylon 6 film and fiber products, we have shown that techniques such as differential scanning calorimetry (DSC), thermomechanical analysis (TMA), and dynamic mechanical analysis (DMA) can reveal the thermal history of the polymer. We believe that the basis of correlation between the heat setting (annealing) conditions and the thermal/mechanical data is the formation of microcrystallites within the amorphous phase upon thermal treatment. Based on a previously published X-ray study, we must mention that a number of changes within the crystalline phase also occur concurrently during the heat setting process of nylon 6. DMA of a highly annealed nylon 6 has helped us identify a broad α-relaxation in the vicinity of 170°C, tentatively assigned to the mobility of interfacial amorphous domains. We propose that the occurrence of α-relaxation parallels the crystalline phase changes observed in X-ray studies during the heat setting of nylon 6 products. Although most of the work presented in this manuscript is based on nylon 6, the proposed correlation between the thermal history and DSC/TMA/DMA techniques can be expected for any semicrystalline polymeric film or fiber.