Corrosion fatigue crack growth rate of a high tensile strength steel HT55 has been measured in 1% NaCl solution at a small ΔK region, where environment, cycle frequency, and load amplitude are changed during fatigue test. Load-strain hysteresis loops are divided into three region of I, II, and III with the boundaries set by a perfect crack closing load and a perfect crack opening one. The load shared by corrosion products is calculated from a shift of the region I. da/dN changes by an environmental alteration from air on 1%NaCl solution or a frequency change under the influence of a corrosion products-induced wedge effect. The region II must be taken into consideration to explain uniquely the corrosion fatigue crack growth behaviors near threshold regions. In the present study the contributory ratio of the region II is known to be 0.54 - 0.35, and a contributory stress intensity factor range ΔKconte is proposed.