Abnormal　crystal　structure　stability　is　discovered　in　the　single-phase　nanocrystalline　Sm2Co17　permanent　magnet.　Three　kinds　of　crystal　structures,　namely　the　rhombohedral　Th2Zn17-type　(2:17　R),　the　hexagonal　TbCu7-type　(1:7　H),　and　the　hexagonal　Th2Ni17-type　(2:17　H),　are　claimed　to　exist　at　room　temperature　in　the　Sm2Co17　alloy　system.　The　strong　dependence　of　the　magnetic　properties　on　the　structure　characteristics　in　the　single-phase　Sm2Co17　alloy　is　interpreted　in　view　of　the　atom　space　occupancy　and　the　exchange　coupling　between　substructures　especially　in　the　nanocrystalline　alloy.