In the present paper we summarize some of our results obtained in the field of magnetic testing of thermal shock fatigue testing of power plant steels. In this work 15Mo3 type ferritic heat resistant steel was investigated. This steel and several similar grades are commonly used in power plants boilers as the material of reheating steam pipelines and pressure vessels. Their typical application temperature is about 500-550 °C. It is commonly accepted that a combined form of mechanical, thermal fatigue and the creep processes causes the long term deterioration of this structural steels. The applied thermal shock fatigue test can model the material degradation due to long term service in high temperature environment. A parallel motion vibrating sample magnetometer, an AC permeameter and a magnetic Barkhausen noise measuring device was used for testing the magnetic properties of the samples and several different magnetic properties were measured to characterize the microstructural processes of the deterioration.