Introducing　dispersive　and　coherent　nanoprecipitates　by　early-stage　decomposition　has　been　recognized　as　a　promising　strategy　to　achieve　super-functional　properties,　such　as　the　large　and　sensitive　magnetostriction　in　Fe-Ga　alloys.　Considering　that　the　diffusion-controlled　decomposition　is　very　sensitive　to　material　position,　where　the　atomic　diffusion　rate　can　be　several　orders　faster　at　material　surface　than　that　at　material　interior,　it　is　necessary　to　investigate　how　the　transformation　depth　affects　the　microstructure　and　the　macroscopic　property.　Here,　we　investigated　the　depth-dependent　phase　transformation　and　magnetostriction　of　isothermally-aged　Fe75Ga25　alloy,　which　evolves　a　diffusion-controlled　phase　transformation　from　metastable　body-centered-cubic　(BCC)　to　equilibrium　face-centered-cubic　(FCC)　intermediated　by　face-centered-tetragonal　(FCT)　phase.　X-ray　diffraction　(XRD)　and　Transmission　electron　microscop　(TEM)　results　revealed　that　from　the　surface　to　the　interior,　both　volume　fraction　and　size　of　the　FCC　phase　gradually　decrease,　together　with　the　gradual　structure　transformation　from　FCC　to　FCT.　When　compared　with　the　solution-treated　state　prior　to　aging,　ordering　of　the　BCC　matrix　also　happens　at　the　sample　interior.　The　gradual　changes　in　precipitates’　structure　and　morphology　result　in　the　gradual　magnetostriction　enhancement　from　the　material　surface　to　the　interior　since　the　FCC　phase　has　negative　magnetostriction　constant,　being　opposite　to　that　of　the　BCC　matrix.　Our　work　not　only　reveals　that　the　over-transformed　surface　layer　of　the　bulk　Fe-Ga　material　should　be　removed　for　fabricating　large-magnetostriction　materials　but　also　suggests　that　the　depth-dependent　microstructure　should　be　carefully　controlled　when　applying　the　precipitation　strategy　to　develop　high-performance　materials.　©　2021　Elsevier　B.V.