This study deals with the microstructure and tensile properties of 600 MPa-grade seismic reinforced steel bars fabricated by a pilot plant. The steel bar specimens are composed of a fully ferrite-pearlite structure because they were air-cooled after hot-rolling. The volume fraction and interlamellar spacing of the pearlite and the ferrite grain size decrease from the center region to the surface region because the surface region is more rapidly cooled than the center region. The A steel bar specimenwith a relatively high carbon content generally has a higher pearlite volume fraction and interlamellar spacing of pearlite and a finer ferrite grain size because increasing the carbon content promotes the formation of pearlite. As a result, the A steel bar specimen has a higher hardness than the B steel bar in all the regions. The hardness shows a tendency to decrease from the center region to the surface region due to the decreased pearlite volume fraction. On the other hand, the tensile-to-yield strength ratio and the tensile strength of the A steel bar specimen are higher than those of the B steel bar with a relatively low carbon content because a higher pearlite volume fraction enhances work hardening. In addition, the B steel bar specimen has higher uniform and total elongations because a lower pearlite volume fraction facilitates plastic deformation caused by dislocation slip.