The　microstructures　and　nano-scale　precipitate　phases　of　Custom　455　stainless　steel　quenched　from　850　degrees　C　and　over-aged　at　480　degrees　C　for　256　h　have　been　studied　in　a　quantitative　way　by　using　transmission　electron　microscopy　and　atom　probe　tomography.　The　results　show　that　plate　reverted　austenite　is　formed　between　the　martensite　laths.　In　the　reverted　austenite,　a　rod-shaped　Ni3Ti　phase　particle　of　about　40　nm　long　is　precipitated　at　the　interphase　interface　between　martensite　and　reverted　austenite,　with　an　adjacent　Cu-rich　precipitate　with　a　size　of　similar　to　20　nm.　In　the　martensite,　spherical　G-phase　particles　with　a　diameter　of　about　8.0　+/-　1.6　nm　are　precipitated　adjacent　to　the　Cu-rich　precipitates　with　a　size　of　about　10.0　+/-　2.0　nm.　These　Cu-rich　precipitates　in　the　martensite　are　much　smaller　than　those　in　the　reverted　austenite,　but　have　a　ten　times　higher　number　density　than　that　in　the　latter.　The　orientation　relationships　between　the　martensitic　matrix,　reverted　austenite,　Ni3Ti　phase,　G-phase　and　Cu-rich　precipitates　are　(01-1)(M)//(1-11)(gamma)//(0004)(eta)//(02-2)(G)//(111)(Cu),　[111]　(M)　//[011](gamma)//[2-1-10](eta)//[111](G)//[11-2](Cu).