This work focuses on investigating fatigue strength of stainless maraging steel, X5CrNiCuNb16-4 (1.4542 according to ÖNORM EN 10088-3). This kind of material achives higher mechanical strength and hardness by heat treatment. These materials are common in automotive and aviation industry. The main task of this thesis was to do rotating bending fatigue tests in the field of high cycle fatigue. A further goal was to test the effect of heat treatment. Notched specimens with two different diameters with and without heat treatment were tested and compared. Additionally, a task was to do simulations with the finite element method. The objective was to develope a simulation model of a rotary bending specimen which considers real testing conditions. Real conditions imply loads, distances on rotating bending machines and the clamp of a specimen. Based on this model a goal was to investigate further simplifications of simulation model parameters and to evaluate, if the results are within a permissible tolerance. In this thesis are results of rotating bending fatigue tests. Specimens of basic material and materials with three different heat treatments were tested. The heat treatments took between 1 and 10.5 hours. The results of specimens with nominal cross-section d=4.00mm show that in the high cycle fatigue area the gradient k of the S-N curve increases with longer heat treatment period. The results of specimens which were heat-treated for 10.5 hours, achieves a gradient of k=12.06. Overall, a heat treatment did not increase the fatigue limit. The results of specimens with a heat treatment period of 1.5 hours show a 14% lower fatigue limit and specimens with a heat treatment which took 10.5 hours achive a 3% lower fatigue limit than basic material. The results of specimens with nominal cross-section d=7.50mm show the same influence of heat treatment on the S-N curve. The gradient k of the heat treated specimens is k=11.53 and the treatment leads to a 14% lower fatigue limit. The conclusion of the simulation is, to simplify the simulation model, which includes real conditions, to a simple model of the notched area of the specimen. A part of optimization of rotating bending machine was to measure real stress of an installed specimen. The deviation from the nominal stress is 2.85% and was taken into consideration in the development of a new Excel sheet for calculating the stress of a bending test specimen.