Todays, the fatigue design follows the concept of the fail-safe design which admits crack that is shorter than the critical crack length. So, the quantitative assessment of fatigue crack propagation life estimated by fracture mechanics plays an important role in fail-safe design. Therefore, the quantitative estimation of a plastic zone, which is the principal factor in fatigue propagation, become an momentous problem. Many studies have been attempted to determine a relation between effective stress intensity range ratio(U) and stress intensity factor(K), remote applied stress(${sigma}$), and crack length(a) but unfortunately the relation of these parameters is not clearly established. In this study, the fatigue tests were conducted for the evaluation of the experimental and analytical evidence for the relation between U and the basic fatigue crack growth parameters. From the test results, the effective stress intensity range(U) is broadly catalogued into three classes and the crack closure effect decreases with increasing applied stress range, and crack length. Accordingly, the crack opening level is dependent on the stress intensity factor(K) which is a function of both applied stress range(${Delta}{sigma}$) and crack length(a). In the low stress intensity range(${Delta}K$), the fatigue strength, which is estimated by Paris-Erdogan laws, is smaller than that evaluated by the consideration of crack closure.