In this thesis, we use IF Steel to be the matrix which composition of carbon is 50ppm approaches Iron and belongs to the part of BCC structure. After we use the load 350kgf precrack, while the propagation rate approaching 2x10-5~3x10-5mm/cycle, we immediately drop the stress intensity factor(£Gk) to a level which is determined by the propagation rate of 9x10-7mm/cycle. Then we record the cycle number as P1, and after the crack tip continues to propagate, we record the cycle number again as P2, and between the period of (P2 ¡V P1), we take 4 sample which marks are 0.25(P2 ¡VP1), 0.5(P2 ¡VP1), 0.75(P2 ¡VP1), (P2 ¡VP1). By this way, we can find out the difference of microstructure of the crack tip from a high load level to a low one. On this experiment, we observation the vein structure, dislocation walls, and so on. It proves that the same phenomenon could also be happened on BCC material, just like FCC material. As time goes by, after drop load, the stored energy is more and more strong. And the vein structure of reverse evolution would become the structure of dislocation cells of normal evolution. Then the crack would continue to propagate on the circumstances.