Under certain harsh conditions in the nuclear industry, austenitic stainless steels need to resistant the intergranular corrosion and also have high strength and toughness. However, the conventional stainless steels such as 310 and 316 steels cannot meet the service requirements. In this work, it focused on the effects of different nitrogen content on microstructure and mechanical properties for an ultra-low carbon austenitic stainless steel in both hot-rolled and solid-solution-treated conditions, which are expect to provide an important reference for a suitable nitrogen content added in the steel. It reveals that the steels were the single austenitic phase and without any δ ferrite when the nitrogen content increased from 0 to 0.5% by XRD analyses. With the increase in nitrogen content, the average grain size of steels decreased gradually. It indicates that the addition of nitrogen can significantly refine the grain size, especially in the solid-solution-treated conditions. However, when the nitrogen content reached about 0.3%, the precipitates began to formed. When the nitrogen content further increasing to about 0.5%, abundant nitrides proved Cr2N and CrN nitrides with relatively larger size were found. The results of mechanical properties indicate that with the increase in nitrogen content, despite of the slight decrease in elongation, the Vickers-hardness, the ultimate tensile strength and yield strength were significantly improved. When the nitrogen content was about 0.3%, the ultimate tensile strength, yield strength and elongation of the solid-solution-treated steel were 798 MPa, 388 MPa and 63.3%, respectively, exhibiting an excellent balance of strength and ductility. The improvement of the strength was mainly derived from the solid solution strengthening of nitrogen, fine-grain strengthening and small portion of precipitation strengthening. Therefore, based on the balance of mechanical properties, it suggests that the nitrogen content should not be higher than about 0.3% in the ultra-low carbon austenitic stainless steel in this work.