Končar – Generators and Motors Inc. is carrying out renovations of the Puntarikoski hydroelectric power plant in Finland. As part of this revitalization, most of the main components of the hydrogenerator are not replaced with new parts. However, because some parts of the hydrogenerator have been in use for more than 50 years, they must be mechanically re-calculated especially with regards to fatigue integrity and lifetime. In this thesis, fatigue integrity and lifetime were assessed for a welded construction of a hydrogenerator, more precisely, the rotor spider of the hydrogenerator, during the load cycle from nominal speed to runaway speed (the maximum speed that occurs during operation). Before assessing the rotor spider of the hydrogenerator, the validity of the software package Autodesk Simulation Mechanical while observing the FKM guideline, was confirmed on a simpler example of two mutually welded plates. The assessment procedure consists of two steps: creating a numerical model using the software package Autodesk Simulation Mechanical, and application of the FKM guideline. The FKM guideline offers two assessment approaches, global and local, and the ability to calculate for non-welded and welded construction elements. The global approach encompasses nominal stress, while the local approach comprises hot spot stress or effective notch stress. The assessment of the rotor spider of hydrogenerator was made for all three stress approaches. It also included calculations of both non-welded and welded construction elements, with the help of two numerical models of the hydrogenerator’s rotor spider, which differ only by modeled welds at the highest stress sites. In this way, a comparison of these stresses was obtained, as were improved calculations of fatigue integrity and lifetime of hydrogenerator parts.