Cyclic tension and compression test was performed to clarify the transition from fatigue fracture to ductile fracture in structural steel. The fracture transition was clearly identified in the plastic strain accumulation pattern, the fractographic appearance, and the crack penetrating profile. When the elongation amplitude a is less than the transition amplitude aTr, the fatigue is predominant, so that the skeleton strain in the stress-strain hysteresis curve approaches zero, striation patterns are observed in the fractographic picture, and the crack goes straight into the thickness without any void around the crack tip. On the other hand, when a is greater than aTr, the ductile fracture is predominant, so that the skeleton strain increases with the elevated amplitude, mashed dimple patterns are observed in stead of striations, and the crack penetrates in zigzags joining coalesced voids.