The peculiarities of the superplasticity effect of the aluminium Al–Zn–Mg–Cu–Zr system alloy with two types of initial grain structure are considered. For the first type of structure, the large grains are predominant, and submicrocrystalline grains take up less volume. Another type of grain microstructure is that of large grains, rounded by small submicrocrystalline grains, which occupy a significantly larger relative volume. Samples with both types of microstructure exhibit superplastic properties, however, the range of flow stresses and strain rates in which the alloy samples are superplastic is much wider for a microstructure where submicrocrystalline grains predominate. For both types of microstructure, deformation processes are mainly localized in those volumes of samples where submicrocrystalline grains were concentrated. However, large grains change their shape during superplastic deformation. The reason for this may be the manifestation of the hydrodynamic flow during the process of deformation.