The　nanocrystalline　SmCo8.9Si0.9　alloy　with　a　high　stability　was　first　prepared　and　then　a　systematic　study　was　carried　out　on　the　phase　transformation　and　corresponding　magnetic　properties　of　the　metastable　SmCo(8.9)Si(0.9　)phase.　The　results　reveal　that　the　addition　of　Si　greatly　improves　the　stability　of　the　metastable　supersaturated　solid　solution　SmCo9.8.　With　the　increase　of　the　heat-treatment　temperature,　the　nanocrystalline　SmCo8.9Si0.9　alloy　transforms　from　the　SmCo9.8　(H)　phase　to　the　Co(fcc)　phase　and　the　coercivity　is　correspondingly　improved.　The　mechanism　is　that　the　precipitated　fine　Co　phase　enhances　the　pinning　effect.　With　a　further　increase　in　temperature,　the　Sm2Co17(H)　phase　transforms　into　Sm2Co17(R)　accompanied　with　an　evident　grain　growth　and　an　inhomogeneous　grain　size　distribution,　decreasing　the　magnetic　properties.