It is important to clarify the effect of cathodic protection (CP) on corrosion fatigue properties of shipstructural steels in the actual environment of ballast tank. In this study, corrosion fatigue tests were conducted on notched base material specimens and welded joint specimens of a high strength steel for shipbuilding in a simulated environment of ballast tank under intermittent immersion condition (IIC) in synthetic seawater (SSW) at 60°C with CP. And the results were compared with those under continuous immersion condition. It was revealed that CP improves the corrosion fatigue life of the steel under IIC, and CP of-1.0 V (vs. SCE) in SSW is remarkably effective, especially in a longer fatigue life region. However, the fatigue lives under IIC both in SSW with CP of-0.8 V and in 3.5% NaCl aqueous solution with CP of-1.0 V are shorter than that in SSW with CP of-1.0 V. It is considered that the improvement in fatigue life under IIC in SSW with CP of-1.0 V is mainly caused by the wedge effect of calcareous deposits which have been produced on the crack surface during the periods of immersion and have been remaining there during the periods of exposure to damp atmosphere. Therefore, an application of linear cumulative damage law using S-N curves for continuous immersion of SSW with CP of-1.0 V and free corrosion condition leads to a very conservative estimation of IIC fatigue life, especially in a lower stress range region.