A method of predicting the longitudinal stresses induced on a ship hull in sea waves is proposed. In short-and long-term sea waves, the total normal stress on a longitudinal member can be evaluated by using the variances of three component sea loads ; vertical wave bending moment, horizontal wave bending moment and wave axial force, and the covariances between those component loads, when the warping stress could be ignored. Exact application of this method is made for a gigantic oil tanker and an approximate longterm correlation method is examined for the same ship in comparison with the so-called square root method. Satisfactory results have been obtained by the long-term correlation method though the square root method would be available for the practical purpose.