In compliance with the ecological and economic requirements it is necessary to improve the efficiency of machines and equipment. According to economic aspects it is very important to raise the endurance of the products, which are under very high cycle load. To reach this aim, it is indispensable to have profound knowledge of the material’s fatigue behaviour, when very high cycle must be achieved. This master thesis deals with the material behaviour of a stainless steel by cycles over 10^7. Fatigue tests were carried out to define the material’s fatigue strength by different test diameters and stress ratios. Specimens with diameters of D4 = 4 mm and D7,5 = 7,5mm were tested by stress ratios of R = −1 and R = 0. The statistical evaluation of the test points was handled with a special evaluation tool. In the low cycle fatigue range the evaluation tool showed two suitable distributions, on the one hand a normal distribution and on the other hand a weibull distribution. In the range of the endurance limit the evaluation was carried out according to the arcsin√P transformation. The fatigue limit at a stress ration of R=0 of specimens D4 showed a decrease of 18% compared to the fatigue limit at a stress ration of R=-1. The comparison of different specimen sizes, D4 and D7,5, showed an increase of the fatigue limit of 4% from the larger specimen geometry to the smaller specimen geometry. As the test results showed, fractures took place in case of cycle loads under the fatigue limit. Investigations of the fracture surface showed that the type of fracture can be divided into two groups. In the first group, which represents the low and high cycle fatigue (LCF, HCF), the crack origin is at the surface. The second group which represents ultra-high cycle fatigue (UHCF), showed a crack origin within the subsurface. In orders to include the fatigue tests into a simulation tool, it is indispensable to have knowledge how the software tools deal with the different influence factors. Therefore the simulation tool FEMFAT® was compared with the simulation tool fe-safe®. The results of this master thesis showed the influence of the geometric size and the mean stress effect on the endurance limit of a stainless steel and the applicability in a commercial fatigue analyse software.