Physical simulation model of γFe → αFe grain nucleation & growth in isothermal solid-state transformation and the rule of grain nucleation & grain growth’s boundary migration are established. The Genetic Algorithm is introduced in the process of grain nucleation & growth. The grain boundary migration agrees with the theory of grain growth. The system free energy ΔG calculation formula is exported. The thermodynamic criterion of new nucleus growing up is that new grain average radius rc is bigger than theory critical average radius rc0 when ΔG is achieved ΔGmax, ΔG is continually decreased at the remaining transformation, the final energy ΔGf is less than initial energy at the end of transformation; be influenced by the limit of nucleation number N.