Fatigue strength is one of the most important factors to be taken into consideration in the design of ships. When analyzing the fatigue strength of ship hull structures, it is necessary to obtain both the long term distribution and the time history of wave-induced loading. Recent investigations have found that short term distribution over a short period, about one hour, is closely approximated by the Rayleigh distribution, while long term distribution, which consists of a large number of short term random condition, has not yet been completely clarified. Up to now, designers have assumed it to be an exponential distribution. The time history of wave-induced loading during actual sailing condition has not been made clear at all. The purpose of this study is to clarify the two not well known factors mentioned above by analyzing data collected from 38 ships that sailed the North Pacific over a period of 14 years (1976-1989). The main points made in this study are as follows. 1) Various wave conditions were recorded in the same sea zone during the same period to clarify their scatter band. 2) The distribution function of long term wave-induced loading can be analytically described by the Weibull distribution function, with an expected shape parameter value close to 1.0 3) The time history of wave conditions during real sailing can be divided into two types. One is calm sea condition, and the other consists of various storm conditions. These two kinds of conditions appear altermately in random order.