Abstract: The dynamic response of an elastic-plastic cantilever beam subjected to step loading at its tip was studied numerically by employing large deflection governing equations and solving them expressed in the finite-difference form. The deformation mechanism and energy dissipation were explored by analyzing the instantaneous distribution of internal force, deformation and energy during the early dynamic response and were compared with those of rigid-plastic analysis. Numerical calculation indicates that the deformation mechanism is different due to the magnitude of the step loading. Elastic-plastic analysis confirms the validity of the rigid-plastic theory in dealing with the case of moderate load, and it points out the defects of the rigid-plastic method in the case of low and intense dynamic loads. Comparison with the results calculated by the small deformation theory shows that it is required to consider the large deformation effects.