The effect of stress ratio, frequency and temperature on the corrosion fatigue crack growth rate of high strength steel has been examined in a 3.5% sodium chloride solution. Corrosion fatigue crack growth data (da/dN - ΔK) observed in the middle range of stress intensity are well expressed by the parallel curve of the data obtained under the conditions of the laboratory air. It is found that the parallel data obtained between these two environments are the results of the reduction of fatigue fracture toughness in corrosive environment. The decreased fracture toughness value is named as an environmental fatigue fracture toughness, Kfe. The hydrogen permeating into the plastic zone at the crack tip is thought to be the main reason for this toughness reduction. It is shown that the Kfe successfully explains the acceleration of fatigue crack growth rate in aqueous environment.