AbstractMechanical and rheo‐optical data have been obtained as a function of frequency at constant temperatures for two series of spherulitic and nonspherulitic samples of isotactic polypropylene having different ordering on molecular level. It was found that the structure differentation influence on the nature of the α relaxation process. In the case of samples containing large spherulites, the α relaxation process is mainly attributed to the friction phenomena between boundaries of the crystalline lamellae and to a certain contribution arising from the intralamellar regions accompanied by restricted reorientation of crystalline elements within orienting lamellae. However, the presence of a smectic phase as well as the decrease of both the spherultie size and the structure perfection leads to enhancement of the mobility of crystallites, and consequently to an increase of the contribution from the intramellar regions to the α relaxation process. Two components of the α relaxation process were found for nonspherulitic samples, crystallized from the glassy state. The low‐temperature component αa(below 80°C) appears at low frequencies (less than 1 Hz) and is mainly attributed to the stress relaxation of the fraction of the noncrystalline phase containing strained molecules and segments of molecules belonging to the specific (irregular) arrangement of the surface layer of the crystallites. The high‐temperature component of the α relaxation process, αa, appears above 80°C and is connected with the viscous slip process of the crystalline elements within the noncrystalline phase. The internsity of this process increases with the decrease of crystallite size. The presence of the smectic phase in the nonspherulitic structure enhances the extent o