The statistical predictions of vertical wave bending moment were made for a gigantic oil tanker of 310 meters length and the similar ships of different lengths, in order to find out the long-term trends of vertical wave bending moment and to examine the design values of those. Firstly, the response functions of vertical wave bending moment were calculated by the aid of the modified strip theory. Then, the standard deviations of vertical wave bending moment for different short-term sea conditions were evaluated based upon the linear superposition theory, by using the calculated response functions and the formulated wave spectra for given sea conditions. Finally, the long-term predictions of vertical wave bending moment were determined by using the obtained standard deviations of vertical wave bending moment for the individual short-term sea conditions and the wave statistics of the North Atlantic Ocean. According to the systematic prediction works, the longitudinal distributions of vertical wave bending moment predicted with the exceeding probability of 10^-8 are investigated in relation to the ship speed and heading angle, and the predicted results were compared with the design values of vertical wave bending moment given by the different classification societies.