The influence of a substitution of Mn for Fe in the Sm2(Fe,Mn)17Nx alloys on the structure and the magnetic properties was studied. Sm2(Fe,Mn)17Nx alloy powders of 32–74 μm in particle size with several values of x were successively synthesized using NH3+H2 gas. It was found that the primary phase of the powders maintained the Th2Zn17-type structure in the wide range for 0≪x≪6. On the other hand, the coercivity of the powders remarkably increased for x≫3. The Sm2(Fe0.95Mn0.05)17N5 powder proved to have a coercivity of 0.62 MA/m (7.7 kOe), without a fine grinding. Microstructural analysis by transmission electron microscopy and energy-dispersive x-ray analysis revealed the powder consisted of Sm2(Fe,Mn)17N3 nanocrystalline grains of 10–30 nm in diameter surrounded by Mn-enriched amorphous phase. © 1997 American Institute of Physics.