It has recently been recognized that structural failures of large ocean-going vessels are essentially related to the dynamic stresses induced by bottom and bow-flare slamming in rough seas. Bottom slamming may cause an excessive vertical bending moment in a hull girder which may lead buckling of the upper deck, while bow-flare slamming may cause an excessive torsional moment in the fore-body of high speed ships in oblique waves. Local dents and cracking failures due to slamming are also observed in the fore body of large ships, they might be accelerated by severe corrosion. In high speed ocean-going ships, cracking failures are sometimes observed in middle and aft parts of the hull girders after passing through a rough sea. Since this type of failure is not necessarily exceptional to newly build ships, it may be caused by low cylce fatigue due to repeated whipping stresses induced by bow-flare slams. In the present paper, fatigue strength is investigated for structural members with high stress concentration factors in the neighbourhood of the front end of the superstructure of a container ship, where the applied stress is composed of a wave induced bending stress including non-linear effects, whipping components of stresses, and possibly coupled vibratory stresses of the superstructure. Having calculated the dynamic stress components among regular waves in head seas, the fatigue strength of highly stressed portions are examined by using appropriate S-N curves, in which effects of ship speeds and wave heights are taken into considerations. When a high speed vessel passes through very rough seas, the fatigue strength may be reduced due to the significant elevation of the repeated stress range caused by the whipping components of stresses superposed on the wave induced bending stress.