A new experiment method called hole filling method was proposed to simulate the internal cracks inside rolled pieces. The healing behavior of internal cracks in a mild steel plate during rolling was studied by presetting internal cracks in a sample. The crack healing tests in the center and 1/3 height of the plate were carried out. The results showed that internal cracks heal completely at the reduction of more than 50% and the rolling temperature 1100°C. The crack in the center of the plate healed better than that at other places, and cooling slowly after rolling was good for healing. The mechanism of crack healing in the rolling processes was analyzed. During the rolling deformation, the compressive stress state provides the static condition for crack healing. When the plastic deformation went on, as the huge atoms move towards one direction, which provided the material supply condition for crack healing, the atoms on two sides of the crack are easy to find a stable location to form a crystal lattice, then the crack heal.