For the purpose of fabricating ultra-high nitrogen austenitic steels (>1mass%N), the phenomenon of nitrogen absorption into solid solution was thermodynamically analyzed and applied to Fe-Cr-Mn system ternary alloy. During the annealing of the steel in a nitrogen gas atmosphere of 0.1MPa at 1473K (nitrogen absorption treatment), the nitrogen content of the steel was increased with the absorption of nitrogen gas from the material surface and then saturated when the system reached a state of equilibrium. The effect of the steel composition on the equilibrium nitrogen content was formulated taking account of interactions among Cr, Mn and N atoms, and the condition for fabrication of ultra-high nitrogen austenitic steels was clarified. The nitrogen addition to ultra-high content markedly increased proof stress and tensile stress of the austenitic steels without losing moderate ductility. For example, Fe-24Cr-10Mn-1.43N (mass%) alloy has 830MPa in 0.2% proof stress, 2.2GPa in true tensile stress and 75% in total elongation.