Abstract: In order to study the microscopic mechanism of strengthening under low amplitude loading,microstructure was investigated using transmission electron microscopy.The specimens were got from truck front axle.After strengthened under low amplitude loading,the wall of dislocation was formed along the grain boundary,resulting in the increases of the friction near the grain boundary when the grain was deformed,and closer combining of the cells.The lattice constants were kept constant under cycled high load.Those explained the mechanism of strengthening under low amplitude loading microscopically.