In the case of analyzation of longitudinal strength of ships, still water bending moments have a major part of total loads. Their statistical natures during actual voyages have not been clarified sufficiently. Moreover the long-term distribution of wave loads is affected by statistical character of draft and trim in service. It is important to clear above natures in order to analyse reliability of longitudinal strength of ships. Authors study the natures of still water conditions by survaying log-books of 10 container ships and 13 tankers. The results of our investigation indicate that their distributions are described by normal distribution approximately. In the case of container ships, they are nearly full load conditions and coefficient of variation of still water bending moment is about 30%. As for tankers, coefficient of variation of still water bending moment at ballast condition is about 100% and that of full load condition is about 35%. These results mean that the variability of still water load of container ships is not larger than that of tankers. So tankers are required to have larger safety margine in order to keep the same strength reliability as that of containers. This is contrary to present concept of load conditions of these ships.