As a preliminary study of developing 800MPa tensile strength grade structural steel plates, the ferrite grain refinement was investigated in a plain low carbon steel. Steels with the composition of 0.15%C-1.1%Mn-0.25%Si were heavily deformed in the uniaxial compression mode and cooled down by using a Gleeble 1500 hot deformation simulator. When a specimen was deformed by 80% at a strain rate of 10/s at 775℃ which was higher than Ar3 temperature of the undeformed specimen by 10℃ and then cooled to room temperature at 10℃/sec, a fine microstructure comprising small ferrite grains of about 2㎛ in size and cementites precipitating at grain boundaries was developed. This compressed specimen showed a high Vickers hardness of 210 which could be converted to tensile strength of 720MPa. Ferrite grains exhibited a random crystallographic orientation and misorientation angles between two neighboring grains were mostly larger than 15 degrees. Some grains divided by subgrain boundaries were also observed. From various experimental results, it could be concluded that these fine ferrites were formed by deformation induced ferrite transformation and dynamic recrystallization of ferrite. Ferrite grains formed by heavy deformation grew very fast during holding time after compression. It was also found that ferrite grains became coarser with decreasing strain rate.