A super martensitic stainless steel was alloyed with nitrogen, and afterwards subjected to quenching and partitioning treatments. Then the performance of three super martensite stainless steels without and with 0.23% and 0.35% nitrogen (in mass fraction), respectively were characterized by means of OM, SEM, TEM, EBSD, BSD, universal testing machine and Vickers hardness tester, in terms of the effect of nitrogen on their microstructure and mechanical properties. The results show that: The addition of nitrogen refines the martensitic slats of the steels, and of which the average width was reduced from 2.93 μm to 0.65 μm. During the partitioning treatment, the high nitrogen concentration provides a driving force for the formation of inverting austenite and facilitates its stabilizing at room temperature. The strength and plasticity of the steels alloyed with nitrogen are obviously higher than those of the nitrogen free ones, the tensile strength and elongation of steels with 0.23 N and 0.35 N are 1510 MPa and 24.2%, and 1215 MPa and 35.1%, respectively. It can be seen that nitrogen alloying is beneficial to improve the mechanical properties of super martensitic stainless steel.