15Mo3 type steel samples were subjected to creep loading at 550°C in order to investigate the development of microstructure as well as the effect of the carbides. It was found that after reaching the secondary creep region, the slope of the creep curve decreased, the creep rate reached a minimum value and remained unchanged as the effect of strainhardening is counterbalanced by an annealing influence. After a short plateau, the slope increased. In order to explain this type of behaviour and to suggest a damage mechanism, TEM and EBSD investigations were performed. At the beginning of the process, subgrain formation started which was retarded by the carbides. At elevated temperature, however, the average distance between carbides increased, and the pinned dislocations were able to bypass them, the internal strain inside the grains (after reaching a maximum value after about 481 hours) decreased, and the creep process accelerated.