The healing mechanism of hydrogen-attacked cracks in low carbon steel and Cr-Mo steel and its influencing factors during the healing process were studied by recovering heat treatment of split specimens in vacuum. The result showed that crack pacing turns much smaller under the condition of pure heating, especially for crack tips. The healing effect is well related to the length of cracks with the shorter in priority. By the primary mechanism of thermal diffusion, iron and carbon atoms must diffuse at the high speed in steel to realize that plasticity deformation energy exceeds and overcomes surface tensile force energy. In addition, phase transformation and stress-stain relationship also have positiwe effects on the process.