In accordance with the Theological and dissipative phenomena in the process of crack propagation, the thermodynamical balance equations of cracked body in the course of crack propagation are established. Then we transform the problem of crack propagation into a problem of diffusion motion which contains the source of mass flow, and investigate the crack extension force by the use of internal body-force field. We transform the problem of energy dissipation into a problem of heat conduction by the introductio...