In geothermal power plants pumps are exposed to corrosive and oscillating loads. This leads to corrosion fatigue and reduces the lifetime of certain components. Usually underwater power pumps are placed inside the borehole. In this case an installation depth of 600 to 800 meters below ground level is typical. In these depths an additional load due to high pressure and high temperature is introduced which is based on geothermal brine. Therefore the material which is used for the pump is steel.For fatigue testing at loads existing in deep geological layers an appropriate system for fatigue tests under in-situ conditions was set up. The requirements of the design are: temperature of the corrosion medium of 200 °C pressure of 200 bars and the complete immersion of the specimen in flowing corrosion medium containing mixtures of corrosive gasses during the entire test period.The test system has three main components the corrosion chamber the heating module and the reservoir. It is important that the corrosion chamber is only fixed onto the specimen which allows for highest flexibility of the test system operational in many different test machines. The heating module heats the corrosion medium up to 200 °C. The reservoir make sure that the volume of the corrosion medium satisfies requirements of the DIN 50905 Part 1 is conformed providing minimum required corrosive medium of 10 ml/cm² in relation to the sample surface.This innovative fatigue test system aims at performing in- situ fatigue corrosion tests simultaneously at high pressure and high temperature in liquid and gaseous corrosive environment. Results from the tests will allow determining the fatigue life of several materials influenced by pressure and temperature under corrosive conditions.