Abstract: For the practical metallic material, preceding the fracture, there always exist the domains of plasticity. For the plasticity limited in scope, the linear elastic mechanics is still applicable, but it is neccesary to correct the influence of the domain of plasticity. The traditional method of correction (Irwin's method) is to introduce the conception of the effective fracture length, namely, owing to the existence of the domain of plasticity, the practical length of the fracture increases. If we take the length of the fracture to be equal to the effective fracture length 2(a+r_y), where 2a is the original length of the fracture and 2r_y is the added length, we may not regard the existence of the domain of plasticity and still use the linear elastic fracture mechanics to deal with.In this paper, we regard that owing to the existence of the domain of plasticity, the practical fracture length and the extremal applied stress both increase, i.e., the value of the two parameters a and σ₁ (external applied stress) both change.In this paper, it is pointed out that the stress intensity factor being determined by the method in this paper eq. (3.2) is closer to Duffy's experimental eq. (3.6) coinciding with experiment than the commonly used eq. (3.4).