A method of predicting the non-linear stresses such as the von Mises' equivalent stress on the longitudinal members of a ship in sea waves is proposed, by assuming that total normal stress and total shearing stress would be regarded as completely dependent randam variables. The application of this method is made for a large oil tanker in short-and long-term sea waves. The short-and long-term trends of the equivalent stresses on the longitudinal members are investigated in relation to heading angle.