The thermal aging of Fe-Cr alloys was simulated using atomistic kinetic Monte Carlo techniques. The study was performed varying the Cr content in the range of 12–18 at. % Cr and at temperatures within the miscibility gap, where alpha - alpha prime phase separation occurs. The evolution of the phase-separation process was characterized in terms of precipitate shape, composition, density, and mean size. The results offer a description of alpha - alpha prime phase separation in its early stage, which is hardly accessible to experiments and of key importance in understanding the change in mechanical properties of Fe-Cr alloys under thermal aging. The critical size for a stable precipitate was estimated from the simulation data in the framework of Gibbs’s homogeneous nucleation theory. The obtained results are compared, whenever possible, with available experimental data and the reliability, as well as the shortcomings, of the applied method is discussed accordingly. Despite strong oversimplifications, the used model shows good agreement with experimental data.