Rational design of ship's longitudinal strength should, be based on accurate evaluation of bending moment and shearing force. The nonlinear strip theory and model experiments reveal that the internal force by waves in rough seas differ from the ones obtained by the linear theory. Rules of classification societies are based on the long term prediction of structural responses, which is performed under the assumption of linearity in response and uses of the linear superposition method. According to the recent studies, the effect of nonlinearity becomes to be taken into consideration in the rules of classification societies. In the present paper, calculations are performed for a fine container ship and for a fullbodied bulk carrier using the nonlinear strip theory. Numerical results are obtained for regular waves of various wave height, wave length and wave encounter angle and for irregular waves of ISSC spectra by the nonlinear strip theory, and are compared with the result of O. S. M. : the linear theory. Results obtained are as follows : 1) Sagging moment increases as wave height increases by the effect of nolinearities, and the moment is about 170% of the one obtained by O. S. M. at the wave height 8 m in regular head sea for the container ship. 2) On the contrary, the effect of nonlinearity in hogging moment is not so significant as in sagging moment. Therefore, long term prediction of hogging moment is obtained appropriately by the linear theory. 3) Approximate prediction of long term response of bending moment is performed taking into account of the nonlinearity, and compared with the present rules.