A significant difference in properties and structure of Fe-Ga alloys in their equilibrium and metastable states limits their application. This necessitates an immediate understanding of the importance of cooling rate in the formation of alloy metastable and equilibrium structures. In this paper, we experimentally studied the structure of the Fe-(23−24) Ga alloys after both (i) cooling with different cooling rates and (ii) isothermal annealing between 300 and 575 °C up to 1800 h to report the alloy structure and present the Time- Temperature-Transformation (TTT) diagrams for the metastable to equilibrium transition in these alloys. The critical cooling rates were defined with respect to the formation of the equilibrium state. A theoretical model along with corresponding Monte Carlo simulations for modeling phase transitions and construction of TTT diagram for the same alloys were developed: simulated phase diagrams were analyzed with respect to our the experimental findings.