Fatigue tests on fillet welded joints were carried out using test specimens simulating reduced plate thickness and reduced fillet weld due to corrosion wastage to investigate the influence of corrosion wastage on the fatigue strength of fillet welded joints. The results of the tests and the related studies are summarized below. It was observed that in cruciform fillet welded joints, the tensile fatigue strength of test specimens with reduced plate thickness decreased considerably compa'red to the tensile fatigue strength of sound specimens. In side fillet welded joints, the shearing fatigue strength of test specimens with reduced plate thickness decreased considerably compared to the shearing fatigue strength of sound specimens. Stress analysis of test specimens was carried out using solid model. The maximum stress arised in the root of the fillet weld at the central part of the width of the plate in cruciform fillet welded joints, and also in the root of the fillet weld at the end of center side of the test specimen in side fillet welded joints. The position where the maximum stress arised almost coincided with the position where the crack occurred in fatigue tests. Rules related to thickness of web plate and leg length of fillet weld of hold frames in large bulk carriers were amended in view of the marine casualties to these ships. From the results of fatigue tests, it was concluded that considerable improvement may be anticipated, as a result of the amendments to rules, in the fatigue strength of fillet welded joints in which corrosion wastage has progressed. From the results of studies on measured data of plate thickness, it was observed that the variation in initiation life and progress rate of corrosion wastage in structural members of hull was considerable in several cases. Accordingly, it is very important to carry out timely maintenance after carefully studying and evaluating the status of corrosion wastage in order to prevent hull damage due to corrosion wastage.