Results presented in this paper contribute to investigation of the granulation mechanisms of γ+(Fe,Mn)3C eutectics in the austenite matrix composites (abbreviated EAMC). The specimens corresponding to the nominal composition of eutectic with controlled RE(Ce)-Mg agent modifier additions have been unidirectional solidified with a constant growth rate of 2.18μm/s at a fixed temperature gradient of 800K/cm using vertical Bridgeman method. With the RE-Mg agent modifier, the transition of solid/liquid (S/L) interface from columnar to dendrite (CDT), refinement and developed branching of γ and (Fe,Mn)3C phases in the eutectics, and the transition of growth style from faceted-nonfaceted (F/NF) to nonfaceted-nonfaceted (NF/NF) for γ and (Fe,Mn)3C phases in the eutectic have been observed and investigated theoretically. Those can explain the granulation of γ+(Fe,Mn)3C eutectics in the as cast because the roundness increases with the developed lateral branching of primary austenite dendrites, refinement of e Results presented in this paper contribute to investigate of the granulation mechanisms of γ + (Fe, Mn) 3C eutectics in the austenite matrix composites (abbreviated EAMC). The specimens corresponding to the nominal composition of eutectic with controlled RE (Ce) -Mg agent modifier additions have been unidirectional solidified with a constant growth rate of 2.18 μm / s at a fixed temperature gradient of 800 K / cm using vertical Bridgeman method. With the RE-Mg agent modifier, the transition of solid / liquid (S / L) interface from columnar to dendrite (CDT), refinement and developed branching of γ and (Fe, Mn) 3C phases in the eutectics, and the transition of growth style from faceted-nonfaceted (NF / NF) to nonfaceted- for γ and (Fe, Mn) 3C phases in the eutectic have been observed and investigated theoretically. Those can explain the granulation of γ + (Fe, Mn) 3C eutectics in the as cast because the roundness increases with the developed lateral branching of primary austenite dendrite s, refinement of e