This paper presents several properties of heat source fields accompanying the highcycle fatigue of a dual-phase steel grade. Heat sources were derived from thermal data provided by an infrared focal plane array camera. An especially developed image processing estimates separately the thermoelastic coupling source amplitude and the mean dissipation per cycle. Our experiments underline that dissipation sources are heterogeneous and this forms the beginning of the fatigue test. They also point out a linear evolution of the mean dissipation per cycle as a function of the loading frequency for a given stress range and a given loading ratio.