Analysis on fatigue lives under random service loading conditions has been extensively attemped to apply the design of offshore structures, ship hulls, pressure vessels and other various structures for the evaluation of the safety in service of their components. At present, Miner's cumulative law is used in most cases for the prediction of fatigue lives in spite of its large deviations and inaccuracies. The principal obstacles in the accurate prediction of fatigue lives are the interaction effects between sequential loading cycles which may cause retardation or acceleration of crack growth. The authors previously analyzed the fatigue crack growth retardation following single, multiple and intermittent overloads and proposed a semi-empirical model for the evaluation of these crack growth retardation processes using a concept of effective stress intensity factor range. In the present report, experimental studies were conducted on the fatigue crack growth under block programed loading, as the second stage of whole the present studies on random loading. The results of experiment showed that the fatigue crack growth behavior under block programed loading was much influenced by the block size and number of cycles in maximum stress range. In case where the block size is small, the interaction effects is relatively small. But the degree of retardation becomes larger as the block size increases. A semi-empirical model which was deduced from the retardation model was proposed for the prediction of these crack growth behavior. The life estimation by this model agree well with the experimental results.