The as-quenched Fe-9Al-28Mn-1.8C (in wt.%) alloy was directly gas-nitrided at 500 C for 8 h, resulting in a ∼30 μm-thick nitrided layer. The nitrided layer consists predominantly of nano-crystalline AlN with a small amount of Fe4N. The nitrogen concentration at surface was extremely high up to ∼17 wt.% (∼41 at.%). Consequently, the surface microhardness (1700 Hv), substrate hardness (550 Hv), ductility (33.2%) and corrosion resistance in 3.5% NaCl solution of the present gas-nitrided alloy are far superior to those obtained previously for the optimally gas-nitrided or plasma-nitrided high-strength alloy steels, as well as martensitic and precipitation-hardening stainless steels. Moreover, it is very novel that the nitrided layer almost remained coherent and adhered well with the matrix after tensile test. Additionally, the present gas nitriding appeared to overcome the edge effects commonly encountered in plasma nitriding treatments for metals.