Abstract: This paper focuses on the localized nonlinear buckling of pressure vessel under internal pres-sure caused by initial geometric imperfection. The localized plastic buckling occurred in the transition region in the torispherical end closure of a pressure vessel has been analyzed by FEM. By introducing two types of initial geometrical imperfections, the arc-length method of modified-Riks/Ramm procedure is employed to simulate the deformation process during loading. The first type of imperfection is determined from the elastostatic displacements, into the zone where it is circumferentially compressed. The second type of imperfection is the irregular thickness of the vessel, also into the zone where it is circumferentially com-pressed. The buckling point is captured from the load-deflection curve of a typical point within the buckled zone, and the corresponding buckling load is calculated. The results show that after the first buckling initiated, the succeeding loading will lead to more wrinkles within the compressive transition region, which agree well with the results of experiments.