In the previous paper, we showed that logarithmic curve of da / dN Δ K_RP was linear even in the range of stage I as well as stage II of a long crack. Moreover we developed the simulation model of fatigue crack opening and closing phenomena in an arbitrary stress distribution field for the purpose of obtaining RPG load. By using the simulation model, effects of stress ratio and delayed retardation on fatige crack propagation are quantitatively assessed. In this paper, for the purpose of understanding the physical meaning of Δ K_RP , the relation between Δ K_RP and the zone size (ω) which is smaller size between tensile plastic zone at maximum load and compressive plastic zone at minimum load in one loading cycle is investigated by using the simulation model. As a result, it becomes clear that Δ K_RP represents indirectory the zone size 63 recieving fatigue damage ahead of a crack. On the other hand, the equivalent distribution stress is introduced for the estimation of fatigue crack propagation life in complex elements, which is difined as stress distribution in an infinitely wide plate giving the same K value in the elements. Then fatigue life is assessed by using the simulation program inputting the equivalent distribution stress. Estimated fatigue life is good agreement with experimental one under constant amplitude loadings and storm model loadings. However estimated fatigue life is longer than experimental one under overload condition.