The　nanocrystalline　SmCo2　bulk　alloy　with　an　average　grain　size　of　about　22　inn　was　prepared　by　combining　high-energy　ball　milling　and　spark　plasma　sintering　(SPS),　in　preparation　the　amorphous　alloy　powder　obtained　by　milling　was　densified　and　crystallized　during　the　SPS　process.　The　microstructure,　crystal　structure　and　phase　structure　of　such　SmCo2　were　determined.　These　results　show　that　the　nanocrystalline　SmCo2　is　not　able　to　exist　stably　as　a　single　phase　at　room　temperature　and　decomposes　into　SmCo3　and　Sm9Co4　phases,　different　from　the　conventional　coarse-grained　SmCo2　phase,　which　exists　stably　at　room　temperature　Phase　stability　and　transformation　behavior　have　been　explained　by　the　nanoscale　thermodynamic　simulation.　Comparing　with　the　coarse-grained　SmCo2　with　very　weak　magnetic　properties,　the　nanocrystalline　SmCo2　exhibits　remarkable　permanent　magnetism　and　good　overall　magnetic　properties.