Long-term predictions of vertical wave shearing force were made for a gigantic oil tanker of 310 meters length and the similar ships of different lengths, in order to examine the design values of vertical wave shearing force given by the different classification societies. In the first place, the response functions of vertical wave shearing force were calculated by the aid of the modified strip theory which was applied for cases in oblique waves. Then, the standard deviations of vertical wave shearing force were evaluated for different short-term sea conditions, based upon the linear superposition theory, by using the calculated response functions and the wave spectra which were formulated for the different sea conditions. Finally, the long-term predictions of vertical wave shearing force were performed by using the obtained standard deviations of vertical wave shearing force for individual short-term sea conditions and the wave statistics of the North Atlantic Ocean. According to the results of systematic prediction works which were made for a range of ship speed and of heading against wave direction, the longitudinal distributions of shearing force amplitude predicted with the exceeding probability of 10^-6 were obtained and compared with those given by the different classification societies.