Austenitic stainless steels are often used for reactor internals that are subject to mechanical and thermo-mechanical stresses that induce low cycle fatigue (LCF), high cycle fatigue (HCF) and even very high cycle fatigue (VHCF). LCF occurs, for example, in piping materials during heating and cooling cycles in start-up and shutdown operations due thermal stratification. Some nuclear power plant components are additionally subjected to high frequency loading in the HCF/VHCF regime e.g., by thermal striping and flow induced vibrations and their interaction. To investigate the VHCF regime mechanical cyclic loading up to 1010 is required. Therefore, ultrasonic fatigue testing systems (UFTS) with a test frequency of f = 20 kHz are usually used. While LCF and HCF tests are strain or stress controlled, VHCF tests are displacement controlled. Calculation from displacement amplitude to strain/stress amplitude is required. In contrast to high-strength steels and other metallic materials such as aluminium and titanium alloys, displacement-controlled VHCF tests on ASS and UFTS cause serious problems in adjusting the control and test parameters due to the elastic plastic behaviour and, for metastable ASSs, the phase transformation from gamma-austenite into alpha’-martensite. To compare VHCF test results to the ASME standard fatigue curve (total strain amplitude vs. load cycles to failure), a fictitious-elastic and an elastically plastic assessment method was used. The elaborated elastic–plastic assessment method generates good results, while a purely elastic assessment in the VHCF regime, commonly used in literature, leads to significantly nonconservative results. In this work, cyclic loading ap up to the VHCF regime was carried out on AISI 347 and AISI 304L at ambient temperature with load ratio R = -1 using a self-developed UFTS. During fatigue, elastic-plastic deformation occurs in the specimen, which leads to deformation-induced phase transformation and cyclic hardening. The change in the microstructure were investigated in the details using optical and scanning electron microscopy as well as non-destructive magnetic Feritscope measurements.