During last decade, the phase-field method has emerged across many fields in materials science as the powerful tool to simulate and predict the complex microstructure developments. In this study, the microstructure changes of g precipitation in polycrystalline a phase during thermomagnetic treatment in Fe-0.4C alloy are modeled using the phase-field method. Using phase-field simulation, the model reasonably represents microstructure changes in Fe-0.4C system quantitatively. In particular, it is shown that the structural elongation and alignment of the (aϩg) two-phase microstructure is influenced by the anisotropic carbon diffusion induced by the magnetic energy at the initial stage of the g phase formation. Modeling of microstructure developments with the framework of the phase-field method is shown to be a very effective strategy to predict and analyze complex microstructure formation.