As a retardation of fatigue crack propagation occur under two step loading such as high to low loading, it is difficult to predictt the fatigue life in variable loading conditions. Therefore, the mechanism of retardation was investigated by changing the ratio of two step-loading in some materials in this paper. It is found in this study that when the ratio of low loading to high loading is less than 50 %, crack arrest occur in agreement with Elber’s formula about crack closure. We believe that crack closure phenomenon is the main problem in considering mechanism of crack propagation in the second stage. In this paper it is estimated that crack closure phenomenon arises because tensile plastic zone at crack tip is compressed by surrounding elastic zone. On the basis of the crack closure phenomenon, the mechanism of crack propagation in the second stage is proposed.