A two-dimensional cellular automaton model was developed for the simulation of nucleation and growth of ferrite grains at various cooling rates in low-carbon steels. The model calculates the diffusion of the solute and temperature fields in an explicit finite method and incorporates local temperature and concentration changes into a nucleation or growth function, which is utilized by the automaton in a probabilistic fashion. The modeling provides an efficient way to understand how those physical processes dynamically progress and affect nucleation and growth of ferrite grains.