In the previous paper, we showed that logarithmic curve of Δ K _RP da/dN was linear even in the ranges of stages I and II of a long crack. In other words, threshold phenomenon appears only circumstantially due to applying Δ K or Δ K _eff which involve the term not related to the crack extension force such as elastic deformation in addition to the term related to the crack driving force. In this paper, we carried out fatigue crack initiation/propagation test for CT specimens with a notch of root radius 1 mm. Fatigue cracks initiated in the mid-thickness at a notch root as surface cracks. Having considered the change of an aspect ratio of surface cracks before and after coalescence from previous studies for a surface crack propagation and a change of measured RPG load (Re-tensile Plastic zone's Generated load), it was confirmed that the logarithmic curve of Δ K _RP da/dN for a physically small crack of which the length was greater than one microstructural grain size and smaller than a uniform through-thickness crack was in good agreement with that for a long crack. Moreover, for a microstructural short crack which is less than one grain size, it is postulated that dislocations do not exceed beyond barriers such as a grain boundary and a pearlite zone in a ferritepearlite microstructure when loading range appearing tensile plastic zone keeps constant. Then, the equivalent Δ K value is obtained from a plastic zone size. Therefore, from the measured RPG load, equivalent Δ K _RP is defined for a microstructural short crack, relating that Δ K _RP represents compressive plastic zone at minimum load for a long crack. By using equivalent Δ K _RP for a microstructural short crack and Δ K _RP for a physically small crack and a long crack, crack propagation curves are calculated for a crack emanated from a notch root without distinguishing between crack initiation and propagation. The calculated crack propagation curves are in very good agreement with experimental ones. These results indicate that the fatigue life can be prolonged if the grain size is made small. However, we might not be able to expect any drastic improvement if the maximum grain size is greater than about 10 μm. Moreover qualitative studies were also made for the phenomenon about stopping crack and coaxing effect by using the algorithm.