Steel bars used in high-rising building were developed by the addition of V and Nb in medium carbon steels. In this study, two steel bars with different diameters (16 mm and 36 mm) were analyzed via optical and transmission electron microscopy (OM and TEM)., the microstructures of the steels studuied consist of ferrite and pearlite the same as those of the conventional steel bars, but they possess higher yield strengths (over 685 MPa) in combination of considerable elongations (above 10%). The results of transmission electron microscopy reveals that the copious nano-sized (about 20 nm) carbides were interphase-precipitated in ferrite and that the inter-lamellar spacings of pearlite were extra fine, about with a scale of 100 nm. It has been estimated that the small carbides and fine pearlite provide yield strengths, approximately 300 MPa and 800 MPa, respectively. In addition, the volume fraction of ferrite was up to 40%, which offered sufficient soft phase to experience external stress. The results of tensile tests for the steels studied demonstrat that the amount of strain can be up to 1.4% as a yield stress is reached, and the apparent yield point and plateau are present in the stress-strain curves.