Diffusion bonding is an effective method to obtain high fatigue performance in PM steels. The main characteristic of diffusion-bonded materials consists of a well-organized microstructural heterogeneity. This work gives reason of the mechanism underlying this fact by means of microstructural interpretation of dynamic performance. Four point bending fatigue tests have been performed on D.AE and D.HP powder metal steels at density close to 7.1 g/cm³, sintered at 1120 °C (2050°F); cooling rate was 0.8 °C/s (1.4°F/s). Notch sensitivity has been tested with 0.25, 0.9 and 3 mm notch radius.   Particular focus has been made on the interaction of the different phases. The effect of the Cu-rich network surrounding the base powder particles is considered to be the main factor responsible for the high fatigue performance of diffusion-bonded materials.