The long-term predictions of wave bending moment amidships were tried on three kinds of hull forms including cargo ship and tanker, where were assumed the similar ships having different lengths for each hull form in order to investigate the relationship between ship size and wave bending moment. In the first place, the standard deviations of wave bending moment induced on ship hulls in short-term irregular seas were evaluated as functions of average wave period and significant wave height, according to the linear superposition method by using the theoretically calculated response operators and the modified Pierson-Moskowitz type wave spectra (I. S. S. C. -1964 wave spectra). Then, the long-term probabilities of wave bending moment exceeding particular level values were calculated by availing the long-term wave data on average wave period and significant wave height in the North Atlantic. According to the results of long-term prediction, the following trends were concluded on the extreme values of wave bending moment expected to be exceeded with the probability of 10-8 in the North Atlantic : The effective wave height for the midship wave bending moment increases generally with increase of ship length, but takes on unexpectedly large values for ships of about 100 meter length and holds nearly constant for ships longer than about 300 meter.