The phenomenon of crack closure is important in the prediction of fatigue crack growth behavior. Many experimental data indicate crack closures during fatigue crack growths both under tensile–tensile loads and tensile–compressive loads at constant amplitude loading cycles, depending on the magnitude of applied load amplitudes and stress ratios. Appendix A-4300 of the ASME Code Section XI provides two equations of fatigue crack growth rates for ferritic steels expressed by stress intensity factor ranges at negative stress ratios. The boundary of the two equations is classified with the magnitude of applied stress intensity factor ranges, in consideration of the crack closures. However, the boundary value provided by the ASME Code Section XI is not technically well known. The objective of this paper is to investigate the influence of the magnitudes of the applied stress intensity factor ranges on the crack closures. Fatigue crack growth tests using ferritic steel specimens were performed in air environment at room and high temperatures. From the crack closures obtained by the tests, it was found a new boundary which is smaller than the definition given by the Appendix A-4300.