The present paper illustrates the application of infrared thermography measurements for the investigation of fatigue behavior of different types of defects in the high-cycle fatigue range of cast steel 42CrMo4 alloy (1.732). The influence of several defects on the fatigue damage behavior was studied by the combination of fractography and thermography. Different crack initiation times and locations of several defects were detected and evaluated in one specimen. Furthermore, a direct correlation of fractography and thermography was carried out. The presented results demonstrate well that the combination of fractography and thermography can give a significant contribution to the knowledge of crack initiation and propagation modes during ultrasonic fatigue of 42CrMo4 steel.