Abstract: Mechanical behavior and microstructural stability of the high-nitrogen austenitic stainless steel 18%Cr-12%Mn-0.55%N at room/low temperature were investigated with its fracture mechanism discussed. Charpy impact test result showed that the ductile-brittle transformation of the steel specimens takes place at about -70°C and a cleavage-like fracture happens along the {111} facet of austenitic matrix. Furthermore, the tensile test result indicated that a deformation-induced phase transformation happens even at room temperature, from which the products include α' martensite and the other one whose structure and relevant parameters cannot be ascertained yet. Based on both the results, it is found that the steel has a good combination of high strength with favorable ductility when it is used at over -60°C.