Martensite is applied as the main structure of high-strength steel to satisfy the demand for lightweight machines. As the fatigue crack extension life takes almost the whole fatigue life, the complex fatigue extension behavior needs to be further studied. This paper is planned to clarify the effect of the hierarchical microstructures and their interfaces on the fatigue extension and the fatigue crack extension mode for the safe and long-lasting use of this material. To achieve the proposal, the rotating bending fatigue tests of 18% Ni martenstic steel was carried out. Fatigue crack behavior and micostrcture near crack path was observed on the specimen surface. The crack extension was found to be discontinuous and was processed by the sub-crack initiation and coalescence with main crack. By the observation of the microstructure around crack path, the observed sub-crack was found to be the inter-granular crack. The proposed reason for the extension process was thought to be the strain localization by the slip along {110} plane and high angle microstructure intrefcae resistance to dislocation motion. Besides, the crack path included inter-granular and trans-granular crack. And the crack extension mode in this material was thought to be damage accumulation mode.