The effect of boron on the nucleation and growth of ferrite at austenite grain boundaries is examined theoretically under the assumption that the junction of 4-austenite grain boundaries (i.e., the 4-grain junctions) are the dominant nucleation sites of ferrite. Boron segregates to the austenite grain boundaries and reduces the grain-boundary energy; it thereby retards ferrite nucleation at the grain boundary. The retardation is expressed as a decrease in nucleation density due to an increase in the critical activation energy for nucleation, and the calculated value of the fraction of active nucleation sites is in satisfactory agreement with the experimental results. The reduction of the austenite grain-boundary energy, which we obtained by applying the Gibbs isotherm for adsorption to the boron segregation, is of the same order of magnitude as the reduction is deduced from the results of calculations for a decrease in the nucleation density based on experimental results. The growth of ferrite was calculated using DICTRA, which yielded both the volume fraction and the grain size of transformed ferrite as functions of time; the results agreed with the experimental results. This agreement suggests that the influence of boron on the growth rate is negligible. However, the increase in the size of the diffusion cell due to the addition of boron is considered to be the main reason for the slightly larger grain size of ferrite compared with that in boron-free steel; this result is also in good agreement with experimental observations.