Strong　metal-support　interactions　characteristic　of　the　encapsulation　of　metal　particles　by　oxide　overlayers　have　been　widely　observed　on　large　metal　nanoparticles,　but　scarcely　occur　on　small　nanoclusters　(＜2　nm)　for　which　the　metal-support　interactions　remain　elusive.　Herein,　we　study　the　structural　evolution　of　Pt　nanoclusters　(1.5　nm)　supported　on　anatase　TiO2　upon　high-temperature　H-2　reduction.　The　Pt　nanoclusters　start　to　partially　evolve　into　a　CsCl-type　PtTi　intermetallic　compound　when　the　reduction　temperature　reaches　400　degrees　C.　Upon　700　degrees　C　reduction,　the　PtTi　nanoparticles　are　exclusively　formed　and　grow　epitaxially　along　the　TiO2　(101)　crystal　faces.　The　thermodynamics　of　the　formation　of　PtTi　via　migration　of　reduced　Ti　atoms　into　Pt　cluster　is　unraveled　by　theoretical　calculations.　The　thermally　stable　PtTi　intermetallic　compound,　with　single-atom　Pt　isolated　by　Ti,　exhibits　enhanced　catalytic　activity　and　promoted　catalytic　durability　for　CO　oxidation.