The　nano-scale　precipitate　phases　in　Custom　455　stainless　steel　quenched　at　850　degrees　C　and　aged　at　480　degrees　C　for　4　h　have　been　examined　in　detail　by　using　transmission　electron　microscopy　and　atom　probe　tomography.　The　steel　contains　a　large　number　of　small,　spherical　Cu-rich　precipitates　densely　distributed　in　the　martensite　matrix　and　some　rod-shaped　Ni3Ti　precipitates,　adjacent　to　the　Cu-rich　precipitates,　together　with　spherical　Ni-rich　G-phase　particles　with　a　composition　close　to　the　stoichiometry　of　Ni16Si7Ti6　phase.　The　average　size　of　the　G　phase　is　about　4.0　+/-　0.8　nm,　and　the　number　density　is　high　(9.8　x　1023　m-3),　similar　to　that　of　the　Cu-rich　precipitates　(9.6　x　1023　m-3).　The　length　of　Ni3Ti　phase　particle　is　about　15.0　+/-　3.0　nm,　and　the　number　density　is　low　(5.2　x　1022　m-3);　The　orientation　relationships　between　the　Cu-rich　precipitates,　the　Ni3Ti　phase　and　the　martensitic　matrix　are:　[-　1-　12]Cu//[-　1100]Ni3Ti//[21-　1]M,　(111)Cu//(0001)Ni3Ti//(011)M.　For　the　G-phase　precipitates,　the　following　orientation　relationships　between　the　G-phase,　the　martensitic　matrix　and　the　Ni3Ti　phase　exist:　[100]G//[100]M//[-　12-　10]Ni3Ti,　(022)G//(011)M//(0001)Ni3Ti.