The　mechanical　property　and　microstructures　of　the　15-5PH　stainless　steel　aged　at　500　degrees　C　for　different　times　have　been　studied　in　detail　by　using　transmission　electron　microscopy　and　atom　probe　tomography.　The　precipitation-hardening　effect　is　significant　during　aging　and　the　peak　hardness　occurs　at　2　h,　but　prolonged　aging　decreases　the　hardness　significantly.　The　peak　hardness　is　attributed　to　the　formation　of　a　large　number　of　Cu-rich　precipitates　of　3-6　nm　in　diameter,　with　the　Ni　and　Mn　atoms　segregating　around　the　Cu-rich　precipitates　and　forming　a　core-shell　structure.　The　over-aging　process　increases　the　size　of　Cu-rich　precipitates　and　causes　Si　and　Nb　atoms　to　segregate　to　the　vicinity　of　Ni　and　Mn　atoms,　forming　Nb-rich　and　NbN-rich　particles.　The　formation　of　various　phase　precipitates,　including　possible　G-phase　particles,　and　their　effects　on　mechanical　property　are　discussed.