Abstract: It is focused on the study of traditional gas nitriding treatment for pure iron (400μm). Three different nitriding methods have been applied which include nitriding by constant temperature cycle, nitriding by high-low temperature cycle and nitriding by intermediate annealing cycle. These methods were employed to reveal the evolution of Fe-N phase microstructure and the grain boundary diffusion mechanism. By compared with the changes of compound layer in composition and thickness, the gas nitriding process was further optimized. As the nitriding progresses, acicular γ′ phase precipitates preferentially in the surface compound layer and the vicinity of the grain boundary, exhibiting a clear preferred orientation. Annealing after nitriding helps nitrogen atoms to permeate more uniformly into the substrate, reducing the precipitation of acicular γ′ phase and facilitating the formation of ε phase which is thicker and more stable. The ε phase acts as a diffusion path, forming a diffusion network with the grain boundaries.