Single-phase　ultrafine　nanocrystalline　Sm2Co17,　a　candidate　material　for　permanent　magnets,　was　prepared　by　a　novel　simple　route　based　on　a　home-built　＇oxygen-free＇　in　situ　fabrication　system.　The　as-prepared　nanocrystalline　Sm2Co17　has　the　stable　hexagonal　Th2Ni17-type　(2:17　H)　crystal　structure　at　room　temperature,　which　is　distinctly　different　from　the　conventional　polycrystalline　Sm2Co17　having　the　rhombohedral　Th2Zn17-type　(2:17　R)　crystal　structure　at　room　temperature.　Phase　transformations　in　the　nanocrystalline　Sm2Co17　alloy　were　investigated　systematically　by　a　series　of　annealing　experiments.　It　was　found　that,　along　with　the　nanograin　growth　in　the　heat-treatment　process,　the　crystal　structure　of　the　room-temperature　phase　transforms　from　2:17　H　to　2:17　R.　The　magnetization　behavior　of　Sm2Co17　alloys　in　different　structural　states　was　characterized.　The　relationship　between　the　magnetic　performance　and　the　structure　characteristics　was　analyzed.　Understanding　the　phase　transformation　of　Sm2Co17　nanocrystals　facilitates　the　development　of　nanocrystalline　Sm2Co17-type　magnets　for　high-temperature　applications.