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Iron-nitride films were prepared by reactive sputtering, and the effect of annealing treatment on the structures was investigated by means of in-situ electron microscopy and high resolution electron microscopy (HREM). As-deposited films were observed to be a mixed structure of a few ultrafine ε-Fe2-3N particles existing in the amorphous matrix. lt was found that the structurerelaxation in the amorphous occurred at 473 K, and the ultrafine grains began to grow at the higher annealing temperatures. The transition of the amorphous to ε-Fe2-3N was almost completed at 673 K. It is considered that the formation of the ideal ε-Fe3N is originated from the ordering of the nitrogen atoms during the annealing in vacuum. On the other hand, γ'-phase (Fe4N) was seen to precipitation of ε-phase at 723 K. Two possible modes are proposed in the precipitation of γ'-phase, depending on the heating rate and crystallographic orientation relationships. i.e. [121]ε [001]γ, (210)ε(110)γ and [100]ε[110]γ, (001)ε(111)γ. In addition,α-Fe particles were observed to form from the γ'-phase at high temperatures. We assumed that these structural changes are due to the diffusion of nitrogen and iron atoms during the annealing,except for the case of the precipitation of the γ'-phase as depicted above. The results obtained in this work are in a good agreement with the assumption.