Nucleation during phase transformations plays an important role in the crystal structure, the grain size and the texture of the forming product phase, and thus determines the properties of the obtained material. In this study, molecular dynamics simulation is employed to study the heterogeneous nucleation of bcc-phase in fcc iron. It is found that the bcc-phase nucleates at the dislocations in the fcc/fcc grain boundaries in a pseudo-cylindrical morphology. The energy change as a function of the bcc nucleus size conforms to the Cahn's classical model with no energy barrier, and provides interface energies and elastic constants comparable to theoretical calculations and experimental data. Nevertheless, there are aspects that cannot be explained by the classical Cahn nucleation theory, namely the stepwise “fcc→intermediate→bcc” nucleation process, and the aggregation of discrete subnuclei. This noclassical nucleation processes contribute to the decrease of energy barrier and the stabilization of the bcc nucleus.