Abstract: Deformation microstructure of pure iron under hypervelocity impact at a speed of 3 km/s was studied by using optical microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that the deformed microstructure beneath the crater can be classified as three zones: fine crystal, high dense twin and low dense twin. At high dense twinning zone, the plastic deformation mechanism of BCC pure iron under hypervelocity impact is 112 deformation twinning and dislocation slips of 110, 112 and 123. The morphological features of twins under impact are different from the conventional deformation. The stress concentration is caused by the dense dislocations assembled adjacent the twin boundary. The kink or flaw along the twin boundary is resulted by the screw dislocations dissociation at the twin boundary.