Development of high strength and high ductility rebar with obvious yield plateau is a tendency for anti-seismic requirement in building steels. In this work, the basic relationship between ferrite grain size and mechanical property, especially the elongation of yield plateau, was investigated by the combination of cold-rolling and heat treatment. After several passes of cold rolling followed by recrystallization annealing for 5~300 min at 600 ℃, ferrite with grain size between 5.2~40.4 μm was obtained in plain steel plates with a carbon content of 0.1%(mass fraction). Mechanical properties were studied by tensile test. With the decrease of ferrite grain size, not only the strength but also the elongation of yield plateau increases. Base on the scanning electron microscopy observation, the deformation becomes more homogeneous with the decrease of grain size, resulting in a higher elongation of yield plateau. A quantitative relationship between grain size d_α and the elongation of yield plateau δ_L was derived from the combination of Hollomon and Hall-Petch equations. The application range of d_α-δ_L equation was also determined, which is well consistent with the experimental results. The narrow-slit spraying equipment was used to treat the HRB400 rebar, and refined ferrite and a considerable non-equilibrium pearlite were obtained due to high cooling rate in treatment. A novel rebar with yield strength of 600 MPa grade was manufactured successfully in commerce, which also verified the above theoretical analysis.