To estimate the extreme value of combined bending moment resulting from wave bending moment and still water moment is the most important problem on analyzation of longitudinal strength of ships. Authors study on the characters of long term distribution of the combined moment, and clarify the following effects of still water load on the characters. In the case, where distribution of still water moment and wave bending moment are described by normal distribution and exponential distribution respectively, long term distribution of combined moment is approximated by exponential distribution which have the same parameter as that of wave bending moment where values of the moment is high. The resulting characteristic extreme of the moment corresponding Q ( M ) =10^-8, is sum of the value of wave bending moment, mean value of still water moment, and a value which is proportional to variance of still water moment. The distribution of extremes of the moment is equal to that of wave bending moment, but the mean value increases by the same value as that of characteristic extreme value. Compering the above extreme value with the one estimated by regarding still water moment is constant, the difference of these values of a tanker is not negligible small.