Fatigue tests with two-step cyclic loading (Low-High and High-Low stress change tests) were carried out on mild steel and stainless steel specimens with different slip characters. Plastic strain range Δεp changed rapidly with the stress change, and it was investigated from a stand-point of rearrangement of dislocation structures, and compared with the analytical values by dislocation dynamics. The dislocation structures were rearranged easily in the Low-High stress change test. On the other hand, in the High-Low test, the specimen fractured before full rearrangement of dislocation because of surface micro-cracks growth resulting from initial high stress amplitude, but the rearrangement was performed sufficiently by the removal of cracks and by sufficient cycling of second low-stress amplitude. The rate of dislocation rearrangement was expressed clearly by the change of cell structure area and was higher in mild steel because of its ease of cross-slip.