Many ship accidents and casualties are caused by large freak ocean waves. Traditionally, the strength of ships against freak waves is assessed by means of ultimate strength evaluation, assuming quasi-static conditions, but the nonlinear dynamic structural response of ships to freak waves should be considered as well. This paper describes how the strength of a ship can be evaluated in terms of its nonlinear vertical bending moment (VBM). Linear dynamic VBM of a ship, which is derived from hydrodynamics, is calculated using a time-domain strip theory code under freak wave conditions, and the nonlinear dynamic VBM, which is dependent on structural nonlinearity, is calculated using a combination of quasi-static and dynamic nonlinear analyses based on the finite element method (FEM). The nonlinear and linear VBMs are then compared to assess how they differ. Then, the influence of freak wave height and wave speed on the VBMs and deformation is studied.