Abstract When a crack initiates and grows in a plain specimen under constant cyclic load amplitude, fatigue crack growth behavior is not reproducible. The fatigue crack length l0 when the scatter of the fatigue crack growth rate converges is reported by observing the crack growth behavior on the specimen surface. The l0 is reported that it is approximately six times as long as the grain size in carbon steels. However, the crack shape of the inside is not observed and we considered that the three-dimensional irregular fatigue crack front shape affects the fatigue crack growth behavior on the specimen surface. Furthermore, the physical meaning and controlling factors of the l0 is still uncertain. Therefore, in this study, we propose two factors that affect the local fatigue crack growth rate: local microstructural and mechanical factors. The former causes a variation of the three-dimensional fatigue crack front shape, and the fatigue crack front shape synergistically affects the mechanical condition at the crack tip. Then we investigated the stress intensity factor values along the tip of the crack including a part of the locally grown crack front. And we propose a concept of force caused by a stable growth part which prevents local growth parts from growing.