A novel route for the preparation of the single-phased Sm2Co17 nanocrystalline bulk with ultrafine grain sizes was proposed. It was found that the nanocrystalline Sm20Co17 has a hexagonal crystal structure at the room temperature, which shows a different thermal stability from the conventional polycrystalline alloy. The intrinsic coercivity of the nanocrystalline Sm2Co17 with a hexagonal crystal structure was greatly increased as compared with the single-phased polycrystalline alloy with a rhombohedral structure. The microhardness and the elastic modulus of the nanocrystalline Sm2Co17 bulk were increased as high as 1.8 and 2.6 times, respectively, when compared with the polycrystalline parent alloy.