In this paper a power cycling based in-situ reliability testing method is discussed for TIM materials. The material under test is put between the cooling surface of a TO-220 packaged semiconductor device and a cold-plate, like during normal operation while the whole assembly is fixed by a constant force. The heat is generated by powering the device in the package at the junction area. The generated heat is lead through the TIM into the cold-plate. The elevation of the junction temperature is used as a sensor to check the thermal property changes of the TIM. The heating power is cyclically switched on and off, and the junction temperature is measured after each cycle by a thermal transient tester. Slight changes in the junction temperature can be detected which correspond to the structural changes occurring in the heat-flow path. It is verified by the structure functions that the temperature changes originate in fact from the TIM layer.