The　structure　and　thermal　stability　were　studied　for　bulk　nanorcystalline　Sm0.8Tm0.2Co5.2　permanent　magnets　produced　by　spark　plasma　sintering　technique.　X-ray　diffraction　results　show　that　the　magnet　exhibits　CaCu5　crystal　structures.　Transmission　electron　microscopy　observation　indicates　that　the　microstructure　of　the　magnet　was　composed　of　fine　and　uniform　grains　with　an　average　size　of　about　50　nm.　Magnetic　measurement　shows　that　the　Curie　temperature　(T-C)　of　the　magnet　is　983　K,　the　magnetization　and　the　remanence　(Mr)　under　9　T　of　the　magnet　are　71.72　and　47.82　emu/g,　respectively,　and　the　coercivity　(Hci)　of　the　magnet　reaches　as　high　as　25.70　kOe.　Moreover,　the　Mr　of　the　magnet　keeps　almost　invariant　as　the　temperature　increases　from　373　K　to　573　K,　and　the　Mr　and　Hci　temperature　coefficient　in　the　range　of　373　K-773　K　are　-0.082%/K　and　-0.144%/K,　demonstrating　the　excellent　thermal　stability　of　the　magnet.