The High Entropy alloys (HEAs) have been widely studied since 2004. At present, the best described multicomponent system is the equiatomic CrCoFeMnNi alloy (Cantor alloy) which serves as a prototype FCC structured HEA. The goal of this work is to link the macroscopic response of the Cantor alloy to a creep loading with its microstructural evolution. The thesis follows up on a long-term creep experiments, carried out at the Institute of Physics of Materials of the Czech Academy of Sciences. The contribution of Grain Boundary Sliding (GBS) is measured by an AFM topography and by a traditional technique based on surface markers. The evolution of the grain and subgrain structures is investigated using High Resolution EBSD (HR-EBSD) and Transmission Electron Microscopy (TEM). Special post processing techniques of the HR-EBSD data (KAM, GROD and GND calculations) are employed to provide information on a formation of subgrains, that are most often delineated by dislocation walls and thus also describe the evolution of dislocation density. The post-processing methodology of the HR-EBSD data is first tested for 316L stainless steel samples deformed by cyclic loading and Ni based superalloy ERBO-1 deformed by creep. An advanced method of the EBSD indexation, the RVB-EBSD, is employed in the case of the ERBO 1 alloy. The RVB-EBSD provides Angular Resolution (AR) of the order of 0.05°, which is a limit required for the analyses performed in this work. The HR-EBSD is then applied to the Cantor alloy samples subjected to creep. The GBS is shown to contribute significantly to the creep strain accumulation at 973 K. The HR EBSD data and the calculations of KAM and GND maps reveal a microstructure evolution which contradicts to a traditional view of the steady creep regime. Contrary to the traditional scenarios, the subgrain microstructure and the dislocation density of the Cantor alloy do not reach a saturation by the end of the primary creep stage but keep gradually evolving during the secondary stage of creep. The relations between the GBS and the unexpected dislocation density evolution are thoroughly discussed. Furthermore, the HR-EBSD method is put to the test by correlations with TEM related techniques, to ensure a validity and to improve the interpretations of the HR-EBSD data. The HEA and HR-EBSD are discussed considering their perspective and relevance for future academic research.