The microstructural features underneath the fracture surfaces of high-strength alloys experienced very-high-cycle fatigue were further investigated to show the nanograin layer in crack initiation regions under negative stress ratios due to numerous cyclic pressing (NCP) mechanism. The grain size and the thickness of nanograin layer in fine granular area of high-strength steels and rough area of titanium alloys were measured. A normalized quantity d* was proposed to characterize the distribution of the nanograin size. A new schematic to express NCP process was depicted to describe the contacting actions between crack surfaces, which causes the microstructure refinement of the high-strength alloys.