We discovered a new compound Sm₂Fe₁₇N₃ in 1987 through the investigation of the structural and magnetic properties of Fe-N-X ( where X denotes an additive element ). The Sm₂Fe₁₇N₃ is a new type of magnet material which is prepared by introducing nitrogen atoms into the interstitial sites of the Sm₂Fe₁₇ crystal under a nitrogen-containing atmosphere at elevated temperatures. The Sm₂Fe₁₇N₃ shows excellent intrinsic magnetic properties (saturation magne-tization M_s = 1.57 T, the anisotropy field H_a = 21 MA/m and Curie temperature T_c = 746 K). The value of the theoretical maximum energy product, (BH)_max amounts to 490 kJ/m³, being a match for the one of the Nd₂Fe₁₄B. We have succeeded in developing high performance Sm-Fe-N bonded-magnets; compression-molding magnet with (BH)_max of 160 kJ/m³ and injection-molding magnets with 110 kJ/m³ have been fabricated so far. Mn-added Sm-Fe-N compacted powder magnet comprised of coarse powder has also been obtained. The Mn-added magnet has good thermal stability and excellent oxidation-resistance.