Using classical molecular dynamics simulations, we study the crystallization of iron under conditions typical of the Earth's inner core. We report that crystal nucleation proceeds with the formation of a critical nucleus, in which all three main metallic structures are present. We show that the crystallization mechanisms and the polymorph selection process strongly depend on temperature. During the crystal growth at the higher temperature, we observe that the fraction of the body-centered cubic polymorph in the nucleus constantly rises at the expense of the close-packed structures while crystal growth at the lower temperature yields a highly polycrystalline phase. (C) 2011 Elsevier B.V. All rights reserved.