Rapid growth of Fe-Co single phase is accomplished by rapid solidification in a drop tube. (Fe, Co) grains are refined with the decrease of alloy droplet diameters. Energy dispersive spectroscopy analysis indicates that microsegregation in (Fe, Co) single phase becomes lower with the reduction of droplet diameters. The experimental results with theoretical calculation reveal that the microsegregation is efficiently suppressed with the increase of undercooling. The free energies of intrinsic segregation for Fe-30 wt.%Co, Fe-40 wt.%Co and Fe-50 wt.%Co alloys are -47.17, -27.57 and -6.57 kJ/mol, respectively. The dependence of free energy of segregation on composition and undercooling has been deduced.