In the Previous report the authors showed a method of plastic analysis of plate girders in postbuckling subjected to shear force. Herein we present a simplified method to determine the collapsing load of plate girders in postbuckling under pure bending. Namely m_u = m_cr +β(1- m_cr ) where, m_u = M_u / M_Y , m_cr = M_cr / M_Y M_u : Ultimate bending moment M_cr : Web buckling moment. M_Y : Flange yielding moment. β : Effectiveness of section modulus in postbuckling. The results of tests carried out by the authors, as well as the other research organizations are compared with the theory which showed a good agreement. A modified interaction curve for collapsing in postbuckling under bending moment and shearing force was introduced.