In order to elucidate thermal stability of Fe-Fe₂Ti eutectic structure, the initial several structures have been investigated in the changes of coarsening and spheroidization during prolonged annealing under the eutectic temperature. The results are as follows: 1) The rate constant of coarsening and spheroidization was formulated as S^(-n)-S^(-n)_0=k·t, where S is the total area of the interface between α and C(Fe₂Ti) per unit volume, S_0 is initial value and k is the rate constant. 2) The coarsening and spheroidization mechanism was described by Ostwald ripening and controlled by diffusion of Ti-atom in α-phase. 3) The spheroidization rate constant in eutectic lamellar structures was depended upon annealing temperature and showed the Arrhenius relation. The activation energy for spheroidization of lamellar structure was 365 kJ/mole.