Corrosion fatigue crack initiation behavior was investigated in high-tensile strength steels in synthetic sea water. The materials tested were 50 kgf/mm2 high-tensile strength steels, HT50, HT50-TMCP, and HT 50 weldment, and a 80 kgf/mm2 one, HT 80. The tests were performed under rotating bending and plane bending at a frequency of 0.17 Hz with a load ratio of -1. Corrosion fatigue strengths of these steels were monotonously decreased with increasing numbers of cycles. The strengths of the base metal and weldment of HT 50 were similar. Corrosion fatigue cracks were initiated when a pit depth exceeded a threshold value. By assuming the pits as sharp cracks, ΔK values at the deepest points were calculated, and the mechanical condition for crack initiation of these steels was determined from the mean value of ΔK, ΔK*<CF>.