Abstract: Based on the fundamental theoretics of material science, mechanics, thermodynamics and physics, the theoretical formulas for yield stress and elongation of materials have been derived in this paper that they are difficult problems in material science and metal forging technology. Their numerical values are in good agreement with the exper imental results. The research will change the phenomenon that yield stress and elongation are only decided on the basis of experimental result. Therefore, it is important to mechanical analysis and designing machine construction for material science and mechanics.