Electric discharge nitriding in liquid nitrogen was applied to the surface of aluminums to form an aluminum nitriding film. The mechanism of the film formation was explained based on the structure of the formed films and results of the emission spectral by means of an electric discharge. The nitriding process formed re-solidified film layers with columnar structure, and hardness values that were more than 1000 HV. It is also possible to prevent the formation of the heat-affected layer. The Al and N2+ species were observed by measuring the emission spectra during the electric discharge.