Recently,　ferromagnetic　Fe-Ga　composites　prepared　by　aging　the　metastable　body-centered-cubic　(BCC)　precursor　within　the　ordered　face-centered-cubic　(FCC)　L12　phase　regime　have　been　found　to　yield　novel　properties,　such　as　highly　thermal-stable　magnetization,　stress-insensitive　magnetic　permeability,　and　sign-changed-magnetostriction.　Accurate　control　of　the　macroscopic　properties　requires　a　thorough　understanding　of　the　microstructure　evolution　during　the　aging　process.　Here　we　performed　an　aging-time　dependent　study　on　a　Fe73Ga27　alloy　to　investigate　the　transformation　process　and　the　associated　internal　structure　evolution.　The　differential　scanning　calorimetry　(DSC)　measurements　reveal　that　the　BCC　to　L12　transformation　is　diffusion-controlled.　The　detailed　transmission　electron　microscope　(TEM)　investigations　reveal　that　this　transformation　has　also　displacive　feature,　exhibiting　shear-induced　twin-related　L12　variants　with　multilayer　internal　structure　prior　to　approaching　phase　equilibria.　The　twin　boundary　and　sublayer　interface　are　found　to　contain　{111}-L12　stacking　faults.　Especially,　the　stacking　faults　within　individual　L12　variants　disappear　after　long-term　aging.　These　findings　suggest　that　this　transformation　is　not　only　driven　by　the　large　free　energy　difference　between　the　metastable　BCC　and　the　equilibrium　L12　phases　but　also　driven　by　reducing　the　stacking　fault　energy,　which　may　help　to　understand　its　slow　transformation　kinetics　and　to　discover　novel　properties　through　engineering　the　microstructure　of　Fe–Ga　alloys.　©　2020　Elsevier　B.V.