The objective of the paper is to understand the mechanism of early stage crack formation in the low cycle fatigue of a martensitic steel with a complex and hierarchic microstructure. As a result of interrupted test at a total strain range Delta epsilon(t) = 1.2% at room temperature, the evolution of deformation relief represented by extrusions and intrusions was characterized by Atomic Force Microscopy and Transmission Electron Microscopy on the surface and in the bulk of specimen. A mechanism of extrusion-intrusion creation based on the vacancy diffusion induced redistribution of matter and plane slipping was proposed. The extrusion-intrusion pair leads to the martensitic steel cracking in the interfaces of microstructure.