The dynamic buckling of a reactor containment vessel under earthquake load is simulated using the nonlinear finite element method to assess the safety margin of the conventional design. The nonlinear FEM is based on the Mixed Interpolated Tensorial Components shell element proposed by Bathe and elaborated by the authors for sensitivity analysis in transient problems. It is found that the buckling load estimated by the conventional static analysis method is smaller (about five-eighths)than that given by the present dynamic analysis method. Sensitivity of the system parameters such as initial imperfection is also studied, leading to implications for design and quality assurance. The effects of the orientation of the polar crane and the existence of hatches on the dynamic response are also studied.