The　complex　interaction　between　precipitation　and　dislocations　challenges　the　determination　of　the　effective　precipitate　nucleation　temperature　(Tnd)　of　dislocation-bearing　supersaturated　solid　solutions,　in　particular,　for　the　2:17-type　Sm-Co-based　permanent　magnets　that　evolve　gradual　formation　and　dissociation　of　dislocations　during　cellular　precipitation.　In　this　study,　the　early-stage　decomposition　behavior　of　a　solution-treated　Sm25Co50.2Fe16.2Cu5.6Zr3.0　(wt.%)　alloy　with　highly-faulted　2:17H　(hexagonal　Sm2Co17)　was　investigated　using　in-situ　high-energy　synchrotron　X-ray　diffraction　(HES-XRD)　and　ex-situ　high-resolution　transmission　electron　microscopy　(HR-TEM).　The　nucleation　and　growth　of　1:5H　(hexagonal　SmCo5)　precipitate　during　heating　and　isothermal　tempering　were　observed　in　situ,　clarifying　a　dispute　in　2:17-type　Sm-Co-based　magnets.　The　effective　nucleation　temperature　Tnd　of　1:5H　precipitates　was　determined　to　be　∼760　°C,　which　is　much　higher　than　the　starting　transformation　temperature　of　2:17H　to　2:17R　(rhombohedral　Sm2Co17)　phase　governed　by　diffusion-controlled　glides　of　Shockley　partials,　Ta∼240　°C.　TEM　studies　revealed　that　a　pre-aging　treatment　at　550　°C　(far　below　Tnd,　but　above　Ta)　causes　dissociation　of　partial　dislocations　whereas　a　pre-aging　treatment　at　750　°C　(slightly　below　Tnd)　promotes　the　nucleation　of　1:5H　precipitates.　As　a　result,　after　whole-process　heat-treating,　the　final　magnet　with　pre-aging　at　750　°C　possesses　better　magnetic　properties　than　the　one　with　pre-aging　at　550　°C　and　the　one　without　pre-aging.　These　results　reveal　that　upon　heating　thermally-activated　motion　of　dislocations　occurs　prior　to　sufficient　atomic　clustering　into　precipitate　nuclei　in　2:17-type　Sm-Co-based　magnets,　providing　direct　guidance　for　designing　proper　material　processing　towards　high-performance.　©　2022　Acta　Materialia　Inc.