Abstract: Carburized heat treatment of 18 Cr Ni Mo7-6 carburized steel was carried out firstly, then the hardness, residual stress tests, microstructure observation and rotating bending fatigue test of the carburized samples were performed. The effects of inclusions and microstructure on the rotating bending fatigue behavior were studied. The results show that the crack sources of the carburized specimens are distributed within the effective carburized layer, and the size of the crack source initiated at the internal non-uniform microstructure is larger than that of the crack source initiated at the internal inclusions. When the fatigue life of sample is lower than 10⁶ cycles, cracks initiate from the sample surface; otherwise they nucleate at the interior.Meanwhile, it is found that there is a competitive relationship between the size of inclusions and the microstructure. When the microstructure of the surface is refined, inclusions with large sizes are prone to become crack sources, while the non-uniform microstructure becomes crack sources in the coarse microstructure. This research can provide guidance for optimizing the heat treatment process of carburized steel.