The design concept of ocean structures is now changing from the “design by rules” concept to the “design by analysis” concept, where starting from load analysis, various kinds of analysis such as stress analysis and failure analysis are carried out in a same flows. And it can be also pointed out as a special feature of the “design by analysis” that safety of structures is estimated from probabilistic point of view instead of the decisive estimation which has been adopted for long time.In the case of probabilistic estimation of structual safety, it is necessary to handle the two kinds of probability, the probability of structural strength which is originated from randomness of external loads, and the probability of material strength caused by scatterness of material qualities. And in the present report, the former problem is investigated mainly.In order to investigate probabilistic problems of the structural strength, it is firstly required to know stochastic characteristics of external loads, wave loads in case of ocean structures, and working stress induced by the loads. Therefore the stochastic pattern of working stress under wave loads is studied mainly experimentally, based on an experiment carried out for two years using a ship on service. Then, taking the stochastic characteristics of working stress into consideration, strength of fatigue strength which is regarded as a most usual mode of failure is investigated from stochastic point of view. Finally the results obtained are compared with the results presented in such codes as BS and AWS.