We use the molecular dynamics simulation to study iron nanoparticles (NPs) consisting of 4000, 5000, 6000 atoms at temperatures of 300 and 900 K. The crystallization and microstructure were analyzed through the pair radial distribution function (PRDF), the potential energy per atom, the distribution of atom types and dynamical local structure parameters <fx>, where x is the bcc, ico or 14. The simulation indicated that amorphous NP contains a large number of ico-type atoms that play a role in preventing the crystallization. Amorphous NP is crystallized through transformations of f14 > 0 and fbcc = 0 type to bcc-type atoms when it is annealed at 900 K upon 40 ns. The growth of crystal clusters happens parallel with changing its microstructure. The behavior of the crystal cluster resembles the nucleation process described by classical nucleation theory. Furthermore, we found that the amorphous NP has two parts: the core has the structure similar to the one of amorphous bulk, in while the surface structure is more porous amorphous. Unlike amorphous NP, the crystalline NP also has three parts: the core is the bcc, the next part the distorted bcc and the surface is amorphous. Amorphous and crystalline NPs have part core which has the structure not depend on size. Doi: 10.28991/HIJ-2021-02-03-01 Full Text: PDF