For hull structural steels, steel plates with ferrite-pearlite microstructure are usually used. On the other hand, it is clarified by our study, that microstructure change from ferrite-pearlite to optimum ferrite-bainite brings superior resistance of fatigue crack propagation rate. Improvement mechanism as to fatigue crack resistance in ferrite-bainite is discussed here. Fatigue crack propagation rate decreased drastically when crack tip reaches a interface between ferrite grain and bainite grain. This mechanism is found through observation of fatigue crack propagation behavior in clad material consisted of ferrite plates and bainite plates. Another mechanism as to fatigue crack resistance is related to cyclic softening behavior. The cyclic softening means that stress at arbitrary constant strain decrease as cycle loading. The more ferrite-bainite steel shows cyclic softening behavior, the more the material has fatigue crack resistance properties. There are two reasons why the cyclic softening decreases the fatigue crack propagation rate. One reason is strain relaxation near crack tip by thin soft layer along fatigue crack path. The other reason is high resistance to ductile fracture after cyclic straining. It is known that resistance against fracture depends on strength level. In the case of ductile fracture, fracture resistance of work hardening materials decreases after straining because of strength increment. On the other hand, the cyclic softening materials are clarified to be advantageous to ductile fracture after cyclic straining, this type of straining may occur near fatigue crack tip before its perforation. From the results of this research, fatigue life of hull structure might be enlarged by application of newly developed fatigue crack propagation resistance steels.