Abstract: The effects of carbon and nitrogen contents on the characteristics and forming mechanism of precipitates in as-cast and aged 18Mn18Cr high nitrogen steels were investigated by optical microscopy, scanning electron microscopy, electron probe micro-analysis, transmission electron microscopy and thermodynamics calculation (Thermo-Calc). It is found that with the decrease in mass ratio of C to N, the precipitate is Cr₂₃C₆ phase, σ phase and Cr₂N phase in the as-cast steel in turn. The precipitation of Cr₂₃C₆ and Cr₂N phase increases as the C or N content increases. C and N contents influence the solidification mode of the tested steel and the eutectoid decomposition of the unstable ferrite phase. The solidification mode of 18Mn18Cr0. 44N steel is an AF mode, and the eutectoid decomposition reactions corresponding to the C contents of 0. 025% and 0. 16% are δ→σ + γ₂ (0. 025% C) and δ→γ₂ + Cr₂₃ (C_xN_y)₆ (x/y〉 1) (0. 16% C), respectively. The solidification mode of 18Mn18Cr0. 72N steel is an A mode, and a little granular Cr₂N phases precipitate on grain boundaries. Only laminar Cr₂N_{0. 39}C_0.61phase exits in the aged steel, and with the increase of C + N content, the volume fraction and lamellar spacing of the precipitates increase, but the incubation time decreases.