The nano-meso-macro strength analysis of fine- and coarse-grain low-carbon steels fatigued at the total strain amplitudes between 0.4 and 2% was conducted using an ultra-micro, micro and Vickers hardness testers and TEM. The coarse-grain steel cyclically hardened, while the fine-grain steel was cyclically stable. The fatigue strength for the fine-grain steel was superior to that for the coarse-grain steel. TEM observations showed that the dislocation cell was formed in the coarse-grain steel. The cell size decreased from 1.1 to 0.75 μm with increasing the total strain amplitude from 0.4 to 2%. On the other hand, the fine-grain steel showed the new dislocation structure. The 1 μm-fine ferrite grain was maintained under cyclic straining. Hardness measurements showed that the virgin and fatigued coarse-grain steels had the different indent-size dependence of the hardness and that the virgin and fatigued fine-grain steels had the unique indent-size dependence of the hardness. The results of TEM observations and hardness measurements explained that the coarse-grain steel cyclically hardened and the fine-grain steel was cyclically stable. Furthermore, the high hardness value, derived from grain-size strengthening in the ferritic steel, explained the superior fatigue strength in the fine-grain steel. The fine grain strengthening hardness due to the ferrite with the size of 1 μm in the fine-grain steel was higher than that due to the dislocation cell with the size of about 1 μm in the coarse-grain steel. The higher hardness in the fine-grain steel was due to the fact that the resistance against the dislocation motion is stronger for ferrite grain boundaries than for dislocation cell boundaries, resulting in the higher fatigue strength.