Abstract: Internal cracks was introduced into 20,45 and 20CrMnTi steel by compressing drilled hole method,and then the cracked samples were heated at different temperatures for different times.Optical microscope and SEM are employed to examine the healing degree of the internal cracks.And the healing mechanism of the internal cracks by heat treatment were studied.The results indicate that there are three obvious stages during the cracks healing process and the main healing mechanisms of the internal cracks in structural steels are recrystallization and grain growth.The substance foundation of crack healing is atomic diffusion.The raise of strain energy or lattice distortion energy caused by the generation of the crack is the internal driving power of cracks healing.There should be a critical energy factor of crack healing.When the sum of the internal driving power and external driving power is higher than the critical energy factor,the cracks are healed.Otherwise,the crack can not be healed.