In　this　paper,　G-phase　precipitation　and　the　resulting　hardening　effect　on　Fe-20Cr-3Ni-1Mn-3Si　ferritic　alloys　by　the　addition　of　Ti,　Nb,　Ta　and　Zr　were　individually　studied　by　electron　microscopy　and　atom　probe　tomography,　combined　with　thermodynamic　and　first　principle　calculations.　The　high　resolution　scanning　electron　and　transmission　electron　microscopy　observations　confirmed　that　four　kinds　of　Ni16M6Si7　(M　=　Ti,　Nb,　Ta　and　Zr)　Gphase　particles　were　distributed　uniformly　in　the　matrix　of　the　four　alloys　aged　at　560-860　degrees　C.　The　3D-APT　results　revealed　the　formation　of　the　four　different　nanoscale　precipitates,　with　the　highest　number　density　(6.05　x　10(23)m(-3))　and　smallest　radius　(1.64　+/-　0.45　nm)　in　the　Ti　added　alloy.　The　nanoscale　sized　precipitates　stability　(similar　to　3　nm　for　Ti　added　alloy;　similar　to　5　nm　for　Nb　added　alloy　and　Zr　added　alloy;　similar　to　25　nm　for　Ta　added　alloy)　was　attributed　to　their　particular　cube-cube　orientation　relationships,　which　led　to　a　very　low　interfacial　energy.　The　effects　of　the　G-phase　on　precipitation　hardening　and　quasi　steady-state　deformation　resistance　were　examined.　The　560　degrees　C-aging　hardening　experiments　suggested　a　rapid　precipitation　process　of　the　nano-particles.　The　peak　hardness　values　of　the　four　alloys　are　in　the　descending　order:　Ti-　＞＞　Nb　＞　Ta-　＞＞　Zr.　The　660　degrees　C　quasi　steady-state　deformation　experiments　showed　threshold　stresses　of　110　and　140　MPa　in　cases　of　the　Nb　and　Ti　added　alloys,　which　were　higher　than　that　of　the　previously　reported　B2-NiAl　strengthened　steels　and　commercial　heat-resistant　steels.　By　using　the　formation　of　nanoscale　G-phase　precipitates,　a　new　ferritic　steel　family　with　very　high　strength　and　creep　resistance　has　been　proposed.　Further　alloy　optimization　is　in　progress.