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Abstract The fatigue assessment of safety relevant components is of importance for ageing management with regard to safety and reliability of nuclear power plants. For reactor internals, austenitic stainless steels are often used due to their excellent mechanical and technological properties as well as their corrosion resistance. During operation the material is subject to loadings in the Low Cycle Fatigue (LCF) regime due to start up and shut down procedures as well as high frequency loadings in the Very High Cycle Fatigue (VHCF) regime induced e.g. by stresses due to fast cyclic thermal fluctuations triggered by fluid dynamic processes. While the LCF behavior of austenitic steels is already well investigated the fatigue behavior in the VHCF regime has not been characterized in detail so far. Accordingly, the fatigue curves in the applicable international design codes have been extended by extrapolation to the range of highest load cycles (Fig. 1). The aim of the cooperative project of the Institute of Materials Science and Engineering (WKK), Materials Testing Institute (MPA) Stuttgart and Framatome GmbH, Germany is to create a comprehensive database up to the highest load cycles N = 2·109 for austenitic stainless steels. For this fatigue tests on metastable austenitic steel AISI 347 / 1.4550 / X6CrNiNb1810 as well as austenitic welds (Fox SAS 2-A) were performed at an ultrasonic testing system at a test frequency of 20 000 Hz to realize acceptable testing times. In addition, an induction generator was implemented in the test system to investigate the influence of operation relevant temperature of 300 °C on the fatigue behavior. The ultrasonic testing system works under displacement control. Therefore, for reliable statements on fatigue life according NUREG/CR-6909 and using of S-N-curve (total-strain amplitude vs. cycle to failure) a fictitious-elastic and elastically-plastic numerical material model was used for calculation of total-strain amplitudes based on experimental data. The results shown, that at ambient temperature (AT) and 300 °C no specimen failure occurred in the VHCF regime for the base material as well as for the welds. Consequently, for these materials a real endurance limit exists. Additionally, in a continuative test a specimen with a pre-autoclaving period in high temperature water (HTW) of 2500 hours was tested in air at a total strain amplitude of 0.1 % in the VHCF-regime up to number of cycle N = 109 using an ultrasonic fatigue testing system. The chemical composition of the HTW for the pre-autoclaving period is comparable to near operation conditions. Afterwards by using of scanning electron microscope no defects or cracks were detected in the oxide layer.